Ignore submodule commits when fetching over dumb protocols

Without this patch, the code would look for the submodule
commits in the superproject and (needlessly) fail when it
couldn't find them.

Signed-off-by: Sven Verdoolaege <skimo@liacs.nl>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch.c

@@ -46,6 +46,9 @@ static int process_tree(struct tree *tree)
 	while (tree_entry(&desc, &entry)) {
 		struct object *obj = NULL;
 
+		/* submodule commits are not stored in the superproject */
+		if (S_ISDIRLNK(entry.mode))
+			continue;
 		if (S_ISDIR(entry.mode)) {
 			struct tree *tree = lookup_tree(entry.sha1);
 			if (tree)