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.
It is careful by default and refuses to overwrite old info, but if you
want to force everything to be re-read, use the "-f" flag.
Some day I'll make it take individual filenames too. Right now
it's all-or-nothing.
