R1:86fa4fd97f89

[Phabricator] track Mercurial bookmarks for differential diffs

Summary:
This adds all the changes necessary to track the active Mercurial
bookmark for differential diffs. We render both branch and bookmark
information in the branch field of the Differential revison view, as
seen in
https://secure.phabricator.com/file/data/kzpmu3evfkukxdjyxrfz/PHID-FILE-eqorsqupxvwirqi2s5lo/bookmark_differential.jpg

The Arcanist half of this is https://secure.phabricator.com/D2896

Test Plan:
Mostly D2896.

Additionally…