It sets up the normal git environment variables and a few helper
functions (currently just "die()"), and returns ok if it all looks like
a git archive. So use it something like
. git-sh-setup-script || die "Not a git archive"
to make the rest of the git scripts more careful and readable.
This means that the .git/objects/pack directory is also rsync'able,
since the filenames created there-in are either unique or refer to the
same data.
Otherwise you might not be able to pull from a directory that is partly
packed without having to worry about missing objects due to pack-file
name clashes.
