t/helper: merge test-subprocess 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-subprocess.c

@@ -1,7 +1,8 @@
+#include "test-tool.h"
 #include "cache.h"
 #include "run-command.h"
 
-int cmd_main(int argc, const char **argv)
+int cmd__subprocess(int argc, const char **argv)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	int nogit = 0;