fetch-pack: print and use dangling .gitmodules

Teach index-pack to print dangling .gitmodules links after its "keep" or
"pack" line instead of declaring an error, and teach fetch-pack to check
such lines printed.

This allows the tree side of the .gitmodules link to be in one packfile
and the blob side to be in another without failing the fsck check,
because it is now fetch-pack which checks such objects after all
packfiles have been downloaded and indexed (and not index-pack on an
individual packfile, as it is before this commit).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -1243,6 +1243,11 @@ int fsck_error_function(struct fsck_options *o,
 	return 1;
 }
 
+void register_found_gitmodules(const struct object_id *oid)
+{
+	oidset_insert(&gitmodules_found, oid);
+}
+
 int fsck_finish(struct fsck_options *options)
 {
 	int ret = 0;