Fix Mercurial "Last Modified" query

Summary:
To determine when a file was last modified, we currently run `hg log ... -b branch ... file`. However, this is incorrect, because Mercurial does not interpret "-b x" as "all ancestors of the commit named x" like Git does, and we don't care about where the modification happened anyway (we always have a resolved commit as a starting point). I think this got copy-pasta'd from the History query.

Instead, drop the branch-specific qualifier and find the last modification, period.

Test Plan: Mercurial commit views of commits not on the repository's default branch are no longer broken.

Reviewers: DurhamGoode, vrana, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5146