adjust_shared_perm(): leave g+s alone when the group does not matter

Julien Moutinho reports that in an environment where directory does
not have BSD group semantics and requires the g+s to be set (aka
FORCE_DIR_SET_GID), but the system forbids chmod() to touch the g+s
bit, adjust_shared_perm() fails even when the repository is for
private use with perm = 0600, because we unconditionally try to set
the g+s bit.

When we grant extra access based on group membership (i.e. the
directory has either g+r or g+w bit set), which group the directory
and its contents are owned by matters.  But otherwise (e.g. perm is
set to 0600, in Julien's case), flipping g+s bit is not necessary.

Reported-by: Julien Moutinho <julm+git@sourcephile.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -901,7 +901,13 @@ int adjust_shared_perm(const char *path)
 	if (S_ISDIR(old_mode)) {
 		/* Copy read bits to execute bits */
 		new_mode |= (new_mode & 0444) >> 2;
-		new_mode |= FORCE_DIR_SET_GID;
+
+		/*
+		 * g+s matters only if any extra access is granted
+		 * based on group membership.
+		 */
+		if (FORCE_DIR_SET_GID && (new_mode & 060))
+			new_mode |= FORCE_DIR_SET_GID;
 	}
 
 	if (((old_mode ^ new_mode) & ~S_IFMT) &&