archive: fix misleading error message

The error message added by 296743a (archive: load index before
pathspec checks, 2024-09-21) is misleading: unpack_trees() is not
touching the working tree at all here, but just loading a tree into
the index.  Correct it.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

archive.c

@@ -536,7 +536,8 @@ static void parse_treeish_arg(const char **argv,
 		opts.fn = oneway_merge;
 		init_tree_desc(&t, &tree->object.oid, tree->buffer, tree->size);
 		if (unpack_trees(1, &t, &opts))
-			die(_("unable to checkout working tree"));
+			die(_("failed to unpack tree object %s"),
+			    oid_to_hex(&tree->object.oid));
 
 		git_attr_set_direction(GIT_ATTR_INDEX);
 	}