Show usage string for 'git show-ref -h'

Show usage string for 'git show-ref -h'

This only changes the behavior of "git show-ref -h" without any
other options and arguments.

"show-ref -h" currently is short for "show-ref --head", which
shows all the refs/* and HEAD, as opposed to "show-ref" that
shows all the refs/* and not HEAD.

Does anybody use "show-ref -h"?  It was in Linus's original, most
likely only because "it might be handy", not because "the command
should not show the HEAD by default for such and such reasons".
So I think it is okay if "show-ref -h" (but not "show-ref
--head") gives help and exits.

If a current script uses "git show-ref -h" without any other
arguments, it would have to be adapted by changing "-h" to
"--head".

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>