resolve-undo.c: use the right index instead of the_index

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

resolve-undo.c

@@ -188,7 +188,7 @@ void unmerge_index(struct index_state *istate, const struct pathspec *pathspec)
 
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
-		if (!ce_path_match(&the_index, ce, pathspec, NULL))
+		if (!ce_path_match(istate, ce, pathspec, NULL))
 			continue;
 		i = unmerge_index_entry_at(istate, i);
 	}