add git features

[edouard-lopez]

• Display git branch name v1.4.0
• Display * when git repository is dirty v1.3.0
• Display ⇡ when branch is ahead
  (commits to push)
• Display ⇣ when branch is being
  (commits to pull)

[edouard-lopez]

See How to get count of unpublished commit with GitPython?

 repo.iter_commits('BRANCH..BRANCH@{u}')

[edouard-lopez]

Check status of local Python relative to remote with GitPython

 commits_behind = repo.iter_commits('master..origin/master')

and

commits_ahead = repo.iter_commits('origin/master..master')

Then you can use something like the following to go from iterator to a count:

count = sum(1 for c in commits_ahead)