add a function to rename sections in the config

add a function to rename sections in the config

Given a config like this:

	# A config
	[very.interesting.section]
		not

The command

	$ git repo-config --rename-section very.interesting.section bla.1

will lead to this config:

	# A config
	[bla "1"]
		not

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