t/helper: merge test-write-cache into test-tool

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

t/helper/test-write-cache.c

@@ -1,9 +1,10 @@
+#include "test-tool.h"
 #include "cache.h"
 #include "lockfile.h"
 
 static struct lock_file index_lock;
 
-int cmd_main(int argc, const char **argv)
+int cmd__write_cache(int argc, const char **argv)
 {
 	int i, cnt = 1, lockfd;
 	if (argc == 2)