Refer users to git-rev-parse for revision specification syntax.

Refer users to git-rev-parse for revision specification syntax.

The revision specification syntax (sometimes referred to as
SHA1-expressions) is accepted almost everywhere in Git by
almost every tool.  Unfortunately it is only documented in
git-rev-parse.txt, and most users don't know to look there.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>