Add keyboard shortcuts to commit view

I'd like to be able to:
- switch focus between stage/unstage/commit (maybe ⌥⌘1, ⌥⌘2, ⌥⌘3) dwells
- to stage/unstage (maybe ⌘S)
- to toggle amend (maybe ⌥⌘A)

here's the commit that adds stage/unstage shortcuts to GitX(L): https://github.com/laullon/gitx/commit/0e61f37ef3d3ad2471d11a2881e4d7fd1e09cb40