Added git mode support for merging, rebasing, and bisecting

<B> Designates Bisecting
>M< Designates Merging
>R> Designates Rebasing
Merge pull request #2633 from redsunsoft/agnoster-theme-git-mode-addition

Added git support for merging, rebasing, and bisecting to agnoster theme
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh