hghave: add a check about whitelisted filesystem that supports hardlink
This is needed for the test added by the next patch.
#require test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> python contrib/check-config.py