git-config-set: support selecting values by non-matching regex

git-config-set: support selecting values by non-matching regex

Extend the regex syntax of value_regex so that prepending an exclamation
mark means non-match:

	[core]
		quetzal = "Dodo" for Brainf*ck
		quetzal = "T. Rex" for Malbolge
		quetzal = "cat"

You can match the third line with

	git-config-set --get quetzal '! for '

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>