Merge branch 'js/merge-tree-3-trees'

"git merge-tree" has learned that the three trees involved in the
3-way merge only need to be trees, not necessarily commits.

* js/merge-tree-3-trees:
  fill_tree_descriptor(): mark error message for translation
  cache-tree: avoid an unnecessary check
  Always check `parse_tree*()`'s return value
  t4301: verify that merge-tree fails on missing blob objects
  merge-ort: do check `parse_tree()`'s return value
  merge-tree: fail with a non-zero exit code on missing tree objects
  merge-tree: accept 3 trees as arguments

merge.c

@@ -77,7 +77,10 @@ int checkout_fast_forward(struct repository *r,
 		return -1;
 	}
 	for (i = 0; i < nr_trees; i++) {
-		parse_tree(trees[i]);
+		if (parse_tree(trees[i]) < 0) {
+			rollback_lock_file(&lock_file);
+			return -1;
+		}
 		init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
 	}