shared repository: optionally allow reading to "others".

shared repository: optionally allow reading to "others".

This enhances core.sharedrepository to have additionally
specify that read and exec permissions to be given to others as
well.  It is useful when serving a repository via gitweb and
git-daemon that runs as a user outside the project group.

The configuration item can take the following values:

    [core]
	sharedrepository 	 ; the same as "group"
	sharedrepository = true  ; ditto
	sharedrepository = 1	 ; ditto
	sharedrepository = group ; allow rwx to group
	sharedrepository = all   ; allow rwx to group, allow rx to other
	sharedrepository = umask ; not shared - use umask

It also extends "git init-db" to take "--shared=all" and friends
from the command line.

Signed-off-by: Junio C Hamano <junkio@cox.net>