Match the git tree of the child gluster module to the parent git module.

Like "Back to the Future" (the movie), we can only affect the future,
unless we go back to the past (rebasing) to add this feature earlier. So
this should only work for versions that are newer that 0.0.3, since I
don't want to rebase git master.
Silence stderr from 'which' when gluster is not installed yet.