Introduce core.sharedrepository

If the config variable 'core.sharedrepository' is set, the directories

	$GIT_DIR/objects/
	$GIT_DIR/objects/??
	$GIT_DIR/objects/pack
	$GIT_DIR/refs
	$GIT_DIR/refs/heads
	$GIT_DIR/refs/heads/tags

are set group writable (and g+s, since the git group may be not the primary
group of all users).

Since all files are written as lock files first, and then moved to
their destination, they do not have to be group writable.  Indeed, if
this leads to problems you found a bug.

Note that -- as in my first attempt -- the config variable is set in the
function which checks the repository format. If this were done in
git_default_config instead, a lot of programs would need to be modified
to call git_config(git_default_config) first.

[jc: git variables should be in environment.c unless there is a
 compelling reason to do otherwise.]

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