Use libgit2 for retrieving config values

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Added support for /.tgitconfig in project root

This makes it possible to easily exchange settings for bugtraq.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Honor /.tgitconfig also for bare repositories

Signed-off-by: Sven Strickroth <email@cs-ware.de>