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. 
Can a test be created to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!