Git::Clone: support shared repositories

Git supports sharing repositories by users of the same group. It works well
enough if you set up the repository's directory with all the right permission
bits and and clone the repository with the right command-line incantations. But
if you don't, it becomes nightmarishly hard to make an existing repository
shared.

Part of the verbosity of this particular patch is that I wanted to ensure each
exec had an onlyif / unless check so that it doesn't pollute the log when it's
not actually doing anything.

Also got rid of now-dead 'notify-submodule-exec'.

Change-Id: I4c282b0ad14b3493026dead4e3b15f2749d5d22e
Merge "Git::Clone: support shared repositories" into production