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/builtin/reset.c b/builtin/reset.c
index 26ef9a7..77c38f2 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -79,13 +79,13 @@ static int reset_index(const struct object_id *oid, int reset_type, int quiet)
 		struct object_id head_oid;
 		if (get_oid("HEAD", &head_oid))
 			return error(_("You do not have a valid HEAD."));
-		if (!fill_tree_descriptor(desc + nr, &head_oid))
+		if (!fill_tree_descriptor(the_repository, desc + nr, &head_oid))
 			return error(_("Failed to find tree of HEAD."));
 		nr++;
 		opts.fn = twoway_merge;
 	}
 
-	if (!fill_tree_descriptor(desc + nr, oid)) {
+	if (!fill_tree_descriptor(the_repository, desc + nr, oid)) {
 		error(_("Failed to find tree of %s."), oid_to_hex(oid));
 		goto out;
 	}