ostree: introduce PAYLOAD_LINK object type

It will be used by successive commits to keep track of the payload
checksum for objects stored in the repository.

The goal is that files having the same payload but different xattrs can
take advantage of reflinks where supported.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1443
Approved by: cgwalters

src/libostree/ostree-repo.c

@@ -3278,6 +3278,8 @@ list_loose_objects_at (OstreeRepo             *self,
         objtype = OSTREE_OBJECT_TYPE_DIR_META;
       else if (strcmp (dot, ".commit") == 0)
         objtype = OSTREE_OBJECT_TYPE_COMMIT;
+      else if (strcmp (dot, ".payload-link") == 0)
+        objtype = OSTREE_OBJECT_TYPE_PAYLOAD_LINK;
       else
         continue;