attr.c: clarify the logic to pop attr_stack

Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -567,7 +567,9 @@ static void prepare_attr_stack(const char *path, int dirlen)
 
 	/*
 	 * Pop the ones from directories that are not the prefix of
-	 * the path we are checking.
+	 * the path we are checking. Break out of the loop when we see
+	 * the root one (whose origin is an empty string "") or the builtin
+	 * one (whose origin is NULL) without popping it.
 	 */
 	while (attr_stack->origin) {
 		int namelen = strlen(attr_stack->origin);
@@ -587,6 +589,13 @@ static void prepare_attr_stack(const char *path, int dirlen)
 	 * Read from parent directories and push them down
 	 */
 	if (!is_bare_repository() || direction == GIT_ATTR_INDEX) {
+		/*
+		 * bootstrap_attr_stack() should have added, and the
+		 * above loop should have stopped before popping, the
+		 * root element whose attr_stack->origin is set to an
+		 * empty string.
+		 */
+		assert(attr_stack->origin);
 		while (1) {
 			char *cp;