Add git-config-set, a simple helper for scripts to set config variables

Add git-config-set, a simple helper for scripts to set config variables

This is meant for the end user, who cannot be expected to edit
.git/config by hand.

Example:

	git-config-set core.filemode true

will set filemode in the section [core] to true,

	git-config-set --unset core.filemode

will remove the entry (failing if it is not there), and

	git-config-set --unset diff.twohead ^recar

will remove the unique entry whose value matches the regex "^recar"
(failing if there is no unique such entry).

It is just a light wrapper around git_config_set() and
git_config_set_multivar().

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