arc branch

Summary:
Appending differential status, sorting, filtering and coloring git
branches.

I think it turned out rather nicely. On my repository with 70 branches
it takes 1.6s, not terrible, though 1.2s is in the conduit call - seems
like there is potential for optimization.

I didn't end up changing 'arc list', as their semmantics are slightly
different, but I'm open to ideas of consolidating them

Test Plan:

• Tested on both facebook www and arcanist repositories.
• Validated that view-all flag works
• Validated that the ordering is correct
• Validated that the statuses match the differential status.

Reviewed By: epriestley
Reviewers: epriestley
CC: aran, epriestley, slawekbiel
Revert Plan:
sure

Other Notes:

Differential Revision: 497