Squashed 'bundle/vim-git-gutter/' content from commit d4a1a60

git-subtree-dir: bundle/vim-git-gutter
git-subtree-split: d4a1a60f399ee4c6a16263e559dbcca70ed0a84a
Merge commit '4fe1c1dd63b59418a8c12452dfcee5d232459d61' as 'bundle/vim-git-gutter'
Add custom highlights for git gutter