Add branch-history API

[ChrisPenner]

Overview

Resolves this ticket

Adds a list of history hashes for each branch at GET /users/<handle>/projects/<slug>/branches/<branch-name>/history?limit=<limit>&cursor=<cursor>

See transcripts for exact response format, it differs slightly from the provided ticket so I can use my existing paging primitives.

Interesting/controversial decisions

Note:

• We need a total ordering of history hashes, which gets a little squirrelly when you consider merges and such.
  • Git just uses the commit timestamp, but we don't have that luxury. I resolve it by sorting on causal-depth, then commit hash; this means causals from either side of the merge point will be interleaved in kinda funky ways, but it's roughly as good as git can do it. We can choose to reflect the full tree structure of the merges at some point in the future if we desire.

[ChrisPenner]

Simon is currently out on Holidays, and I'm about to be, I'll go ahead and merge this so it doesn't go stale, but happy to make fast-follow-ups and change the API structure if needed, nobody will be integrated against it in the meantime :)