Usage message clean-up, take #2

Usage message clean-up, take #2

There were some problems with the usage message clean-up patch
series. I hadn't realised that subdirectory aware scripts can't source
git-sh-setup. I propose that we change this and let the scripts which
are subdirectory aware set a variable, SUBDIRECTORY_OK, before they
source git-sh-setup.

The scripts will also set USAGE and possibly LONG_USAGE before they
source git-sh-setup. If LONG_USAGE isn't set it defaults to USAGE.

If we go this way it's easy to catch --help in git-sh-setup, print the
(long) usage message to stdout and exit cleanly. git-sh-setup can
define a 'usage' shell function which can be called by the scripts to
print the short usage string to stderr and exit non-cleanly. It will
also be easy to change $0 to basename $0 or something else, if would
like to do that sometime in the future.

What follows is a patch to convert a couple of the commands to this
style. If it's ok with everyone to do it this way I will convert the
rest of the scripts too.

[jc: thrown in to proposed updates queue for comments.]

Signed-off-by: Junio C Hamano <junkio@cox.net>