The code which handles notifications regarding tags was unused, as only
objects listed by git-rev-list(1) are considered, and git-rev-list(1)
never spits out the sha1 of a tag object.
Import the (self-written) git-update-mirror script, which updates clones
of Git repositories and then calls git-notify (in just the same way as a
post-receive hook would be called by Git). The git-notify script is
imported from git://source.winehq.org/git/tools.git (commit: 03d66f34)
and generates notifications on repository changes. We'll use these
scripts for generating our commit e-mails.
