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. 
```ruby
(libexec+"bin").install Dir["contrib/remote-helpers/git-remote-{hg,bzr}"]
bin.env_script_all_files(libexec+"bin", :PYTHONPATH => ENV["PYTHONPATH"])
```