R1:efb0fa739f2e

Make tracked git repositories use an implicit 'origin' remote

Summary:
See T624. I originally wrote this to require an explicit remote, but this
creates an ugly "origin:" in all the URIs and makes T270 more difficult.

Treat all branch names as implying 'origin/'.

Test Plan:
- Pulled and imported a fresh copy of libphutil without issues.
- Browsed various git repositories.
- Browsed Javelin's various branches.
- Ran upgrade script, got a bunch of clean 'origin/master' -> 'master'
conversions.