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