Support git branches & merges.

[Robert Bruce Park]

Bzr is dead and the train needs to support git sooner or later.

Fortunately, all the bzr knowledge is encapsulated in a single class (the Branch class), so it should be relatively straightforward to write a similar class that uses git instead of bzr, and then just add a little bit of magic to determine which to use when.

There's a few unknowns I need to investigate:

1. Has LP's git support reached parity with bzr yet? If not parity, the train needs at least the ability to pass a git MP to lp.load() and access various properties on it, such as merge.target_branch, merge.source_branch etc. Hopefully lplib exposes these in a consistent way otherwise we'll need to write quite the wrapper around the two, which has the potential to become an enormous glue layer.

2. We use 'bzr launchpad-login' as well, git probably won't have anything like that but still we'll need to teach git the bot credentials so we can push to launchpad at all.

3. Probably other things.