git-init-db: initialize shared repositories with --shared

Now you can say

git-init-db --shared

if you want other users to be able to push into that repository.

[jc: info/ and objects/info/ need to be group writable if the
repository is shared --- otherwise packs and refs files cannot
be updated.]

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