pull: add support for tombstone commits

Add a new object type: OSTREE_OBJECT_TYPE_TOMBSTONE_COMMIT that is
used when a commit was intentionally removed.

If the remote repository doesn't use tombstone commits, do not fail on
a missing commit (change 0b795785dd).

When the remote repository uses tombstones, if a commit cannot be
found, check if the tombstone file is present and fail if it is not
present.

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

src/libostree/ostree-core.c

@@ -975,6 +975,8 @@ ostree_object_type_to_string (OstreeObjectType objtype)
       return "dirmeta";
     case OSTREE_OBJECT_TYPE_COMMIT:
       return "commit";
+    case OSTREE_OBJECT_TYPE_TOMBSTONE_COMMIT:
+      return "tombstone-commit";
     default:
       g_assert_not_reached ();
       return NULL;