git prototype package

Take some of the original git package code, add a model, and use etch.

Goal is to prototype:

git clone git@github.com:atom/git.git
cd git
apm install
apm link -d
atom .

cd ../some-dir-with-a-git-repo
atom -d

TODO

right on a file focuses hunk
select hunks with up or down
enter to stage hunk
/ to toggle line selection mode
shift-up or shift-down to expand line selection
a to select lines
enter to stage selected lines
backspace while on a changed file (not the diff) will prompt to discard changes
o on a changed file or hunk/line will open that file in a tab