Revert "welcome my old nice diff colors back"

This reverts commit 0390cf231f666de9a8a3e9417271c75d25a34160.
(Re)add colors for Git diff added/removed lines

Fix chriskempson/tomorrow-theme#261.
Merge pull request #4 from yous/restore-git-diff

Restore git diff
Make Git commit summary bold

Uses the gitcommitSummary property from the gitcommit.vim script.

Fix chriskempson/tomorrow-theme#262.