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
Fix `docker rmi` tab completion

This is exactly what happens on tab completion for docker rmi. This commit
fixes that.

```sh
$ docker rmi <hit TAB>
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
```