Make core.sharedRepository more generic

Make core.sharedRepository more generic

git init --shared=0xxx, where '0xxx' is an octal number, will create
a repository with file modes set to '0xxx'. Users with a safe umask
value (0077) can use this option to force file modes. For example,
'0640' is a group-readable but not group-writable regardless of
user's umask value. Values compatible with old Git versions are written
as they were before, for compatibility reasons. That is, "1" for
"group" and "2" for "everybody".

"git config core.sharedRepository 0xxx" is also handled.

Signed-off-by: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>