tree-walk.c: remove the_repo from fill_tree_descriptor()

While at there, clean up the_repo usage in builtin/merge-tree.c a tiny
bit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/unpack-trees.c b/unpack-trees.c
index 5018990..cfe1c5e 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -840,7 +840,7 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,
 			const struct object_id *oid = NULL;
 			if (dirmask & 1)
 				oid = &names[i].oid;
-			buf[nr_buf++] = fill_tree_descriptor(t + i, oid);
+			buf[nr_buf++] = fill_tree_descriptor(the_repository, t + i, oid);
 		}
 	}