mirroring gitolite to external (non-gitolte) servers

[Sitaram Chamarty]

Someone on IRC wanted a way to mirror gitolite to S3 using jgit, and to
restore from that mirror to a fresh gitolite install.

While I don't have either an S3 account or jgit, the problem seemed
generic enough to at least put something in contrib, so that people can
munge it for their own needs.

https://github.com/sitaramc/gitolite/blob/master/contrib/triggers/file_mirror

(Unfortunately I don't know the person's email address and he does not
stay logged on to irc; I hope he's subscribed to the list)