tests: do not use system hg if it does not have "files" command

Ancient hg does not have "hg files" so test-check-*.t will fail with
"unknown command 'files'":

  $ hg files
  hg: unknown command 'files'
  $ hg --version
  Mercurial Distributed SCM (version 2.6.2)

Test "hg files" and give up using syshg if it does not have "files" command.

tests/helpers-testrepo.sh

@@ -1,27 +1,40 b''
 # Invoke the system hg installation (rather than the local hg version being
 # tested).
 #
 # 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.
 #
 # 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
 }
+
+# Most test-check-* sourcing this file run "hg files", which is not available
+# in ancient versions of hg. So we double check if "syshg files" works and
+# fallback to hg bundled in the repo.
+syshg files -h >/dev/null 2>/dev/null
+if [ $? -ne 0 ]; then
+    syshg() {
+        hg "$@"
+    }
+    syshgenv() {
+        :
+    }
+fi