Update git-sh-setup(1) to allow transparent use of git-rev-parse --parseopt

Update git-sh-setup(1) to allow transparent use of git-rev-parse --parseopt

If you set OPTIONS_SPEC, git-sh-setups uses git-rev-parse --parseopt
automatically.

It also diverts usage to re-exec $0 with the -h option as parse-options.c
will catch that.

If you need git-rev-parse --parseopt to keep the `--` the user may have
passed to your command, set OPTIONS_KEEPDASHDASH to a non empty value
in your script.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>