Fix backwards-incompatible handling of core.sharedRepository

Fix backwards-incompatible handling of core.sharedRepository

06cbe85 (Make core.sharedRepository more generic, 2008-04-16) broke the
traditional setting of core.sharedRepository to true, which was to make
the repository group writable: with umask 022, it would clear the
permission bits for 'other'. (umask 002 did not exhibit this behaviour
since pre-chmod() check in adjust_shared_perm() fails in that case.)

The call to adjust_shared_perm() should only loosen the permission.
If the user has umask like 022 or 002 that allow others to read, the
resulting files should be made readable and writable by group, without
restricting the readability by others.

This patch fixes the adjust_shared_perm() mode tweak based on Junio's
suggestion and adds the appropriate tests to t/t1301-shared-repo.sh.

Cc: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>