The initial fetch of a new repo which has 'upstream' read-only mirroring
configured will cause a spurious error concerning FETCH_HEAD not yet
existing. This silences the error.
i.e., a client fetch will invoke the push, (subject to 'nice' setting),
but you can also force a fetch regardless of last fetch time by running
this command directly on the server:
gitolite ../triggers/upstream fetch <reponame>
Also, use FETCH_HEAD instead of own sentinel file (idea courtesy Luke Lu)
