Option parsing for commands that run outside git repos is still screwy,
as there is no Annex monad and so the flags cannot be passed in. But,
any remaining parameters can be, which is enough for this fix.
Inject the required git-remote-xmpp into PATH when running xmpp git push.
Rest of the time it will not be in PATH, and git won't be able to talk to
xmpp remotes.
