attr.c: make bootstrap_attr_stack() leave early

Thas would de-dent the body of a function that has grown rather large over
time, making it a bit easier to read.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

attr.c

@@ -488,9 +488,11 @@ static int git_attr_config(const char *var, const char *value, void *dummy)
 
 static void bootstrap_attr_stack(void)
 {
-	if (!attr_stack) {
 	struct attr_stack *elem;
 
+	if (attr_stack)
+		return;
+
 	elem = read_attr_from_array(builtin_attr);
 	elem->origin = NULL;
 	elem->prev = attr_stack;
@@ -529,7 +531,6 @@ static void bootstrap_attr_stack(void)
 	elem->origin = NULL;
 	elem->prev = attr_stack;
 	attr_stack = elem;
-	}
 }
 
 static void prepare_attr_stack(const char *path, int dirlen)