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-core.c

@@ -1239,6 +1239,8 @@ ostree_object_type_to_string (OstreeObjectType objtype)
       return "tombstone-commit";
     case OSTREE_OBJECT_TYPE_COMMIT_META:
       return "commitmeta";
+    case OSTREE_OBJECT_TYPE_PAYLOAD_LINK:
+      return "payload-link";
     default:
       g_assert_not_reached ();
       return NULL;
@@ -1266,6 +1268,8 @@ ostree_object_type_from_string (const char *str)
     return OSTREE_OBJECT_TYPE_TOMBSTONE_COMMIT;
   else if (!strcmp (str, "commitmeta"))
     return OSTREE_OBJECT_TYPE_COMMIT_META;
+  else if (!strcmp (str, "payload-link"))
+    return OSTREE_OBJECT_TYPE_PAYLOAD_LINK;
   g_assert_not_reached ();
   return 0;
 }
@@ -2122,6 +2126,7 @@ _ostree_validate_structureof_metadata (OstreeObjectType objtype,
       break;
     case OSTREE_OBJECT_TYPE_TOMBSTONE_COMMIT:
     case OSTREE_OBJECT_TYPE_COMMIT_META:
+    case OSTREE_OBJECT_TYPE_PAYLOAD_LINK:
       /* TODO */
       break;
     case OSTREE_OBJECT_TYPE_FILE: