diff --git a/tests/helpers-testrepo.sh b/tests/helpers-testrepo.sh
--- a/tests/helpers-testrepo.sh
+++ b/tests/helpers-testrepo.sh
@@ -1,19 +1,27 @@
-# The test-repo is a live hg repository which may have evolution
-# markers created, e.g. when a ~/.hgrc enabled evolution.
+# Invoke the system hg installation (rather than the local hg version being
+# tested).
 #
-# Tests are run using a custom HGRCPATH, which do not
-# enable evolution markers by default.
+# We want to use the hg version being tested when interacting with the test
+# repository, and the system hg when interacting with the mercurial source code
+# repository.
 #
-# If test-repo includes evolution markers, and we do not
-# enable evolution markers, hg will occasionally complain
-# when it notices them, which disrupts tests resulting in
-# sporadic failures.
-#
-# Since we aren't performing any write operations on the
-# test-repo, there's no harm in telling hg that we support
-# evolution markers, which is what the following lines
-# for the hgrc file do:
-cat >> $HGRCPATH << EOF
-[experimental]
-evolution=createmarkers
-EOF
+# The mercurial source repository was typically orignally cloned with the
+# system mercurial installation, and may require extensions or settings from
+# the system installation.
+syshg () {
+    (
+        syshgenv
+        exec hg "$@"
+    )
+}
+
+# Revert the environment so that running "hg" runs the system hg
+# rather than the test hg installation.
+syshgenv () {
+    PATH="$ORIG_PATH"
+    PYTHONPATH="$ORIG_PYTHONPATH"
+    JYTHONPATH="$ORIG_JYTHONPATH"
+    unset HGRCPATH
+    HGPLAIN=1
+    export HGPLAIN
+}
diff --git a/tests/test-check-code.t b/tests/test-check-code.t
--- a/tests/test-check-code.t
+++ b/tests/test-check-code.t
@@ -7,7 +7,7 @@
 New errors are not allowed. Warnings are strongly discouraged.
 (The writing "no-che?k-code" is for not skipping this file when checking.)
 
-  $ hg locate -X contrib/python-zstandard -X hgext/fsmonitor/pywatchman |
+  $ syshg locate -X contrib/python-zstandard -X hgext/fsmonitor/pywatchman |
   > sed 's-\\-/-g' | "$check_code" --warnings --per-file=0 - || false
   Skipping i18n/polib.py it has no-che?k-code (glob)
   Skipping mercurial/httpclient/__init__.py it has no-che?k-code (glob)
@@ -33,7 +33,7 @@ New errors are not allowed. Warnings are
 
 Prevent adding new files in the root directory accidentally.
 
-  $ hg files 'glob:*'
+  $ syshg files 'glob:*'
   .editorconfig
   .hgignore
   .hgsigs
diff --git a/tests/test-check-commit.t b/tests/test-check-commit.t
--- a/tests/test-check-commit.t
+++ b/tests/test-check-commit.t
@@ -8,8 +8,8 @@ Go back in the hg repo
 
   $ cd $TESTDIR/..
 
-  $ for node in `hg log --rev 'not public() and ::. and not desc("# no-check-commit")' --template '{node|short}\n'`; do
-  >    hg export --git $node | contrib/check-commit > ${TESTTMP}/check-commit.out
+  $ for node in `syshg log --rev 'not public() and ::. and not desc("# no-check-commit")' --template '{node|short}\n'`; do
+  >    syshg export --git $node | contrib/check-commit > ${TESTTMP}/check-commit.out
   >    if [ $? -ne 0 ]; then
   >        echo "Revision $node does not comply with rules"
   >        echo '------------------------------------------------------'
diff --git a/tests/test-check-config.t b/tests/test-check-config.t
--- a/tests/test-check-config.t
+++ b/tests/test-check-config.t
@@ -31,7 +31,7 @@ Sanity check check-config.py
 
 New errors are not allowed. Warnings are strongly discouraged.
 
-  $ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
+  $ syshg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
   >   $PYTHON contrib/check-config.py
               limit = ui.configwith(fraction, 'profiling', 'showmin', 0.05)
   
diff --git a/tests/test-check-execute.t b/tests/test-check-execute.t
--- a/tests/test-check-execute.t
+++ b/tests/test-check-execute.t
@@ -5,20 +5,20 @@
 
 look for python scripts without the execute bit
 
-  $ hg files 'set:**.py and not exec() and grep(r"^#!.*?python")'
+  $ syshg files 'set:**.py and not exec() and grep(r"^#!.*?python")'
   [1]
 
 look for python scripts with execute bit but not shebang
 
-  $ hg files 'set:**.py and exec() and not grep(r"^#!.*?python")'
+  $ syshg files 'set:**.py and exec() and not grep(r"^#!.*?python")'
   [1]
 
 look for shell scripts with execute bit but not shebang
 
-  $ hg files 'set:**.sh and exec() and not grep(r"^#!.*(ba)?sh")'
+  $ syshg files 'set:**.sh and exec() and not grep(r"^#!.*(ba)?sh")'
   [1]
 
 look for non scripts with no shebang
 
-  $ hg files 'set:exec() and not **.sh and not **.py and not grep(r"^#!")'
+  $ syshg files 'set:exec() and not **.sh and not **.py and not grep(r"^#!")'
   [1]
diff --git a/tests/test-check-help.t b/tests/test-check-help.t
--- a/tests/test-check-help.t
+++ b/tests/test-check-help.t
@@ -23,6 +23,7 @@
 Check if ":hg:`help TOPIC`" is valid:
 (use "xargs -n1 -t" to see which help commands are executed)
 
-  $ hg files 'glob:{hgdemandimport,hgext,mercurial}/**/*.py' | sed 's|\\|/|g' \
+  $ syshg files 'glob:{hgdemandimport,hgext,mercurial}/**/*.py' \
+  > | sed 's|\\|/|g' \
   > | xargs $PYTHON "$TESTTMP/scanhelptopics.py" \
   > | xargs -n1 hg help > /dev/null
diff --git a/tests/test-check-module-imports.t b/tests/test-check-module-imports.t
--- a/tests/test-check-module-imports.t
+++ b/tests/test-check-module-imports.t
@@ -14,7 +14,7 @@ these may expose other cycles.
 Known-bad files are excluded by -X as some of them would produce unstable
 outputs, which should be fixed later.
 
-  $ hg locate 'set:**.py or grep(r"^#!.*?python")' \
+  $ syshg locate 'set:**.py or grep(r"^#!.*?python")' \
   > 'tests/**.t' \
   > -X contrib/debugshell.py \
   > -X contrib/python-zstandard/ \
diff --git a/tests/test-check-py3-compat.t b/tests/test-check-py3-compat.t
--- a/tests/test-check-py3-compat.t
+++ b/tests/test-check-py3-compat.t
@@ -3,7 +3,7 @@
   $ . "$TESTDIR/helpers-testrepo.sh"
   $ cd "$TESTDIR"/..
 
-  $ hg files 'set:(**.py)' | sed 's|\\|/|g' | xargs $PYTHON contrib/check-py3-compat.py
+  $ syshg files 'set:(**.py)' | sed 's|\\|/|g' | xargs $PYTHON contrib/check-py3-compat.py
   contrib/python-zstandard/setup.py not using absolute_import
   contrib/python-zstandard/setup_zstd.py not using absolute_import
   contrib/python-zstandard/tests/common.py not using absolute_import
@@ -22,7 +22,7 @@
   tests/test-demandimport.py not using absolute_import
 
 #if py3exe
-  $ hg files 'set:(**.py) - grep(pygments)' -X hgext/fsmonitor/pywatchman \
+  $ syshg files 'set:(**.py) - grep(pygments)' -X hgext/fsmonitor/pywatchman \
   > | sed 's|\\|/|g' | xargs $PYTHON3 contrib/check-py3-compat.py \
   > | sed 's/[0-9][0-9]*)$/*)/'
   hgext/convert/transport.py: error importing: <*Error> No module named 'svn.client' (error at transport.py:*) (glob)
@@ -38,7 +38,7 @@
 #endif
 
 #if py3exe py3pygments
-  $ hg files 'set:(**.py) and grep(pygments)' | sed 's|\\|/|g' \
+  $ syshg files 'set:(**.py) and grep(pygments)' | sed 's|\\|/|g' \
   > | xargs $PYTHON3 contrib/check-py3-compat.py \
   > | sed 's/[0-9][0-9]*)$/*)/'
 #endif
diff --git a/tests/test-check-pyflakes.t b/tests/test-check-pyflakes.t
--- a/tests/test-check-pyflakes.t
+++ b/tests/test-check-pyflakes.t
@@ -6,7 +6,8 @@
 run pyflakes on all tracked files ending in .py or without a file ending
 (skipping binary file random-seed)
 
-  $ hg locate 'set:**.py or grep("^#!.*python")' -X hgext/fsmonitor/pywatchman \
+  $ syshg locate 'set:**.py or grep("^#!.*python")' \
+  > -X hgext/fsmonitor/pywatchman \
   > -X mercurial/pycompat.py -X contrib/python-zstandard \
   > 2>/dev/null \
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
diff --git a/tests/test-check-shbang.t b/tests/test-check-shbang.t
--- a/tests/test-check-shbang.t
+++ b/tests/test-check-shbang.t
@@ -5,11 +5,11 @@
 
 look for python scripts that do not use /usr/bin/env
 
-  $ hg files 'set:grep(r"^#!.*?python") and not grep(r"^#!/usr/bi{1}n/env python") - **/*.t'
+  $ syshg files 'set:grep(r"^#!.*?python") and not grep(r"^#!/usr/bi{1}n/env python") - **/*.t'
   [1]
 
 In tests, enforce $PYTHON and *not* /usr/bin/env python or similar:
-  $ hg files 'set:grep(r"#!.*?python") and **/*.t' \
+  $ syshg files 'set:grep(r"#!.*?python") and **/*.t' \
   > -X tests/test-check-execute.t \
   > -X tests/test-check-module-imports.t \
   > -X tests/test-check-pyflakes.t \
@@ -21,5 +21,5 @@ contain Python but don't end in .py - pl
 
 look for shell scripts that do not use /bin/sh
 
-  $ hg files 'set:grep(r"^#!.*/bi{1}n/sh") and not grep(r"^#!/bi{1}n/sh")'
+  $ syshg files 'set:grep(r"^#!.*/bi{1}n/sh") and not grep(r"^#!/bi{1}n/sh")'
   [1]
diff --git a/tests/test-contrib-perf.t b/tests/test-contrib-perf.t
--- a/tests/test-contrib-perf.t
+++ b/tests/test-contrib-perf.t
@@ -170,6 +170,6 @@ Check perf.py for historical portability
 
   $ cd "$TESTDIR/.."
 
-  $ (hg files -r 1.2 glob:mercurial/*.c glob:mercurial/*.py;
-  >  hg files -r tip glob:mercurial/*.c glob:mercurial/*.py) |
+  $ (syshg files -r 1.2 glob:mercurial/*.c glob:mercurial/*.py;
+  >  syshg files -r tip glob:mercurial/*.c glob:mercurial/*.py) |
   > "$TESTDIR"/check-perf-code.py contrib/perf.py
diff --git a/tests/test-debian-packages.t b/tests/test-debian-packages.t
--- a/tests/test-debian-packages.t
+++ b/tests/test-debian-packages.t
@@ -1,6 +1,7 @@
 #require test-repo slow debhelper
 
   $ . "$TESTDIR/helpers-testrepo.sh"
+  $ syshgenv
 
 Ensure debuild doesn't run the testsuite, as that could get silly.
   $ DEB_BUILD_OPTIONS=nocheck
diff --git a/tests/test-docker-packaging.t b/tests/test-docker-packaging.t
--- a/tests/test-docker-packaging.t
+++ b/tests/test-docker-packaging.t
@@ -1,6 +1,7 @@
 #require test-repo slow docker
 
   $ . "$TESTDIR/helpers-testrepo.sh"
+  $ syshgenv
 
 Ensure debuild doesn't run the testsuite, as that could get silly.
   $ DEB_BUILD_OPTIONS=nocheck
diff --git a/tests/test-hghave.t b/tests/test-hghave.t
--- a/tests/test-hghave.t
+++ b/tests/test-hghave.t
@@ -1,3 +1,5 @@
+  $ . "$TESTDIR/helpers-testrepo.sh"
+
 Testing that hghave does not crash when checking features
 
   $ hghave --test-features 2>/dev/null
@@ -18,7 +20,10 @@ Testing hghave extensibility for third p
   >   $ echo foo
   >   foo
   > EOF
-  $ run-tests.py $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t
+  $ ( \
+  > syshgenv; \
+  > $TESTDIR/run-tests.py $HGTEST_RUN_TESTS_PURE test-hghaveaddon.t \
+  > )
   .
   # Ran 1 tests, 0 skipped, 0 failed.
 
diff --git a/tests/test-imports-checker.t b/tests/test-imports-checker.t
--- a/tests/test-imports-checker.t
+++ b/tests/test-imports-checker.t
@@ -1,6 +1,7 @@
 #require test-repo
 
   $ . "$TESTDIR/helpers-testrepo.sh"
+  $ syshgenv
   $ import_checker="$TESTDIR"/../contrib/import-checker.py
 
 Run the doctests from the import checker, and make sure
diff --git a/tests/test-install.t b/tests/test-install.t
--- a/tests/test-install.t
+++ b/tests/test-install.t
@@ -159,7 +159,7 @@ path variables are expanded (~ is the sa
   >     print('  %s' % f)
   > EOF
 
-  $ $PYTHON wixxml.py help
+  $ (syshgenv; $PYTHON wixxml.py help )
   Not installed:
     help/common.txt
     help/hg-ssh.8.txt
@@ -168,7 +168,7 @@ path variables are expanded (~ is the sa
     help/hgrc.5.txt
   Not tracked:
 
-  $ $PYTHON wixxml.py templates
+  $ ( syshgenv; $PYTHON wixxml.py templates )
   Not installed:
   Not tracked:
 
diff --git a/tests/test-mac-packages.t b/tests/test-mac-packages.t
--- a/tests/test-mac-packages.t
+++ b/tests/test-mac-packages.t
@@ -1,6 +1,7 @@
 #require test-repo slow osx osxpackaging
 
   $ . "$TESTDIR/helpers-testrepo.sh"
+  $ syshgenv
 
   $ OUTPUTDIR="`pwd`"
   $ export OUTPUTDIR