rev-parse: Add support for the ^! and ^@ syntax

rev-parse: Add support for the ^! and ^@ syntax

Those shorthands are explained in the rev-parse documentation but were not
actually supported by rev-parse itself.

gitk internally uses rev-parse to interpret its command line arguments, and
being able to use these "limit with parents" syntax is handy there.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>