Support branch offset/limit in Mercurial

Summary:

• Support offset/limit as added by D2442, for Mercurial.
• We just list everything and slice, no clear way to do better and this isn't a major perf issue.
• No clear way to easily get by-update sorting, we can implement this by looking up revs if someone asks.
• Bury some of the Git implementation details inside the Git query.

Test Plan:

• Looked at a Git repo, clicked "View All Branches".
• Looked at a Hg repo, clicked "View All Branches".
• Limited page size to 1, made sure it limited properly.

Reviewers: aurelijus, btrahan, vrana

Reviewed By: aurelijus

CC: aran

Differential Revision: https://secure.phabricator.com/D2453