git: patch remote helpers' sys.path to find hg/bzr libs

Neither Bazaar nor Mercurial depend on a brewed Python, yet git-remote-bzr/hg expect to find bzr/hg libs in Python's sys.path. This commit patches the git-remote helpers' sys.path with the location of the bzr/hg libs installed via homebrew. 
@jacknagel I don't mind either way, I just think we need to not ask the user to set PYTHONPATH manually when we can set/edit it ourselves.