Summary:
This diff removes "unset HGRCPATH" to try to run tests in a more
controlled environment. And skip the test if system HG looks broken.
Test Plan: arc unit
Reviewers: simpkins, #mercurial, stash
Reviewed By: stash
Subscribers: net-systems-diffs@fb.com, mjpieters, medson
Differential Revision: https://phabricator.intern.facebook.com/D5263484
Signature: t1:5263484:1497604378:28ec94c33ba77680c8d2325a8fa36457e0eb8c91
