Prompt: display more information for git on the branch name

[narnaud]

Note: this is the same as #52 - I just realized I wasn't using git right.

This patch display more information close to the branch name in the prompt for git repositories:

• display the short sha if htere is no branch name
• display some additional information when: rebasing, merging, cherry-picking, bisecting...
• all the rest is the same as before

Screenshot before:

And the same after:

[lukesampson]

Thanks for this.