t/helper: merge (unused) test-mergesort 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-mergesort.c

@@ -1,3 +1,4 @@
+#include "test-tool.h"
 #include "cache.h"
 #include "mergesort.h"
 
@@ -22,7 +23,7 @@ static int compare_strings(const void *a, const void *b)
 	return strcmp(x->text, y->text);
 }
 
-int cmd_main(int argc, const char **argv)
+int cmd__mergesort(int argc, const char **argv)
 {
 	struct line *line, *p = NULL, *lines = NULL;
 	struct strbuf sb = STRBUF_INIT;