Make thin-pack generation subproject aware.

When a thin pack wants to send a tree object at "sub/dir", and
the commit that is common between the sender and the receiver
that is used as the base object has a subproject at that path,
we should not try to use the data at "sub/dir" of the base tree
as a tree object.  It is not a tree to begin with, and more
importantly, the commit object there does not have to even
exist.

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

builtin-pack-objects.c

@@ -979,6 +979,8 @@ static void add_pbase_object(struct tree_desc *tree,
 	int cmp;
 
 	while (tree_entry(tree,&entry)) {
+		if (S_ISGITLINK(entry.mode))
+			continue;
 		cmp = tree_entry_len(entry.path, entry.sha1) != cmplen ? 1 :
 		      memcmp(name, entry.path, cmplen);
 		if (cmp > 0)