perf: add test for writing the index

A performance test for writing the index to be able to
determine if changes to allocating ondisk structure help.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-write-cache.c

@@ -0,0 +1,23 @@
+#include "cache.h"
+#include "lockfile.h"
+
+static struct lock_file index_lock;
+
+int cmd_main(int argc, const char **argv)
+{
+	int i, cnt = 1, lockfd;
+	if (argc == 2)
+		cnt = strtol(argv[1], NULL, 0);
+	setup_git_directory();
+	read_cache();
+	for (i = 0; i < cnt; i++) {
+		lockfd = hold_locked_index(&index_lock, LOCK_DIE_ON_ERROR);
+		if (0 <= lockfd) {
+			write_locked_index(&the_index, &index_lock, COMMIT_LOCK);
+		} else {
+			rollback_lock_file(&index_lock);
+		}
+	}
+
+	return 0;
+}