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>

environment.c

@@ -18,7 +18,7 @@ int log_all_ref_updates = 0;
 int warn_ambiguous_refs = 1;
 int repository_format_version = 0;
 char git_commit_encoding[MAX_ENCODING_LENGTH] = "utf-8";
-int shared_repository = 0;
+int shared_repository = PERM_UMASK;
 const char *apply_default_whitespace = NULL;
 
 static char *git_dir, *git_object_dir, *git_index_file, *git_refs_dir,