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.

• Tried to specify an explicit remote in a default branch name.
• Unit tests.

Reviewers: nh, jungejason, btrahan

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T624

Differential Revision: https://secure.phabricator.com/D1269