Make 'arc diff X' in mercurial match git by using the GCA of X as the base

Summary:
Previously 'arc diff X' with mercurial meant to use X as the base
to diff against.  Now it means use gca(X,working directory) as the base to
diff against.  This matches the git behavior.

Test Plan:
Ran 'arc diff master' on a repo where master was ahead of the feature branch.
Verified that the diff result included only the diffs in the feature branch.

Reviewers: epriestley, sid0, bos, dschleimer

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4865