Introduce GIT_TEMPLATE_DIR

Instead of passing --template explicitely to init-db and clone, you can
just set the environment variable GIT_TEMPLATE_DIR.

Also make use of it in the tests, to make sure that the templates are
copied.

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