R1:d24c81c5e27b

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".