repo-config: trim white-space before comment

Earlier, calling

git-repo-config core.hello

on a .git/config like this:

[core]
hello = world ; a comment

would yield "world " (i.e. with a trailing space).

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>
(cherry picked from c1aee1fd8d94da9b3c5d2dc1d4264f7e73a58f80 commit)