git-describe: default to HEAD
git-describe: default to HEAD

This is based on the patch by Andreas Ericsson, but done slightly
differently, preferring to have separate loops -- one for options
and then arguments.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git