Document --since and --until options to rev-parse.
Document --since and --until options to rev-parse.

The usability magic were hidden in the source code without being
documented, and even the maintainer did not know about them ;-).

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