Gita

* add `pull` subcommand
* bug fixes and refactors

* add `push`, `merge`
* add short commit message

* add `fetch` option
* `gita fetch`: fetch all remote updates
* `gita fetch <repo-name>`: fetch remote updates for the specified repo
* add more color coding to the repo branch name

* `gita add <repo-path>`
* `gita rm <repo-name>`
* `gita ls`
* `gita ls <repo-name>`