adjust_shared_perm: chmod() only when needed.

When widening permission for files and directories in a 'shared'
repository for a user with inappropriate umask() setting for
shared work, make sure we call chmod() only when we actually
need to.

The primary idea owes credit to Johannes.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

path.c

@@ -279,7 +279,7 @@ int adjust_shared_perm(const char *path)
 			    : 0));
 	if (S_ISDIR(mode))
 		mode |= S_ISGID;
-	if (chmod(path, mode) < 0)
+	if ((mode & st.st_mode) != mode && chmod(path, mode) < 0)
 		return -2;
 	return 0;
 }