zsh git repository info

Add the current branch to the prompt line.

TODO: Add some more information such as working tree and index state.
Tried adding remote branches, but it didn't look very good.
Squashed 'emacs.d/themes/solarized/' content from commit ef11b01

git-subtree-dir: emacs.d/themes/solarized
git-subtree-split: ef11b0187547434a3ded91fb0a324c7d7f986ca0
Merge commit '5d8bd93e219be100719ab22062f88baab35b3c26' as 'emacs.d/themes/solarized'