bash_completion: do not modify git $PS1 prompt behavior

Referring to functions defined elsewhere causes annoying noise when
said-elsewhere is not actually available. Also this code for detecting
the repo manifest branch never worked correctly. It simply lists all
branches with the m/ remote prefix, not the current one being used.
This has always annoyed me but I didn't know what was doing it till now.
Merge pull request #16 from marineam/git

bash_completion: do not modify git $PS1 prompt behavior