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. 
Two things: git itself uses libexec already so we should probably choose a different directory for this, and I don't think we should write wrapper script + move the things that git normally installs into bin.