Introduce a new revision set operator <rev>^!

Introduce a new revision set operator <rev>^!

This is a shorthand for "<rev> --not <rev>^@", i.e. "include
this commit but exclude any of its parents".

When a new file $F is introduced by revision $R, this notation
can be used to find a copy-and-paste from existing file in the
parents of that revision without annotating the ancestry of the
lines that were copied from:

	git pickaxe -f -C $R^! -- $F

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