Git prompt for Bash

Summary:
Prints the branch name and some dirty/avancement indicators.

This contributes to T635 and fixes the Git part.

Test Plan:
Rebuild a container and run arc shell, then add a non tracked file.

[root@9b5152be65ca workspace (git-prompt)]# touch quux
[root@9b5152be65ca workspace (git-prompt %)]#

Reviewers: dereckson

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Differential Revision: http://devcentral.nasqueron.org/D135