tests: use system hg only if changelog or dirstate can't be read

The bundled hg should work flawlessly in most cases. Make it depend on
the external installation only if necessary since we can't control the
whole environment.

This patch doesn't implement the "exit 80" idea proposed by Jun. I don't
want to keep the capability checking sync with the actual tests.

tests/helpers-testrepo.sh

@@ -1,54 +1,53
-# Invoke the system hg installation (rather than the local hg version being
-# tested).
+# In most cases, the mercurial repository can be read by the bundled hg, but
+# that isn't always true because third-party extensions may change the store
+# format, for example. In which case, the system hg installation is used.
 #
 # 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 () {
     . "$HGTEST_RESTOREENV"
     HGPLAIN=1
     export HGPLAIN
 }
 
 # The test-repo is a live hg repository which may have evolution markers
 # created, e.g. when a ~/.hgrc enabled evolution.
 #
 # Tests may be run using a custom HGRCPATH, which do not enable evolution
 # markers by default.
 #
 # 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
 
-# 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 [ $? -eq 0 ]; then
+# Use the system hg command if the bundled hg can't read the repository with
+# no warning nor error.
+if [ -n "`hg id -R "$TESTDIR/.." 2>&1 >/dev/null`" ]; then
     alias testrepohg=syshg
     alias testrepohgenv=syshgenv
 else
     alias testrepohg=hg
     alias testrepohgenv=:
 fi