read-cache: be strict about "changed" in remove_marked_cache_entries()

remove_marked_cache_entries() deletes entries marked with
CE_REMOVE. But if there is no such entry, do not mark the index as
"changed" because that could trigger an index update unnecessarily.

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

read-cache.c

@@ -510,6 +510,8 @@ void remove_marked_cache_entries(struct index_state *istate)
 		else
 			ce_array[j++] = ce_array[i];
 	}
+	if (j == istate->cache_nr)
+		return;
 	istate->cache_changed = 1;
 	istate->cache_nr = j;
 }