read-cache: add simple performance test

Add the helper test-read-cache, which can be used to call read_cache and
discard_cache in a loop as well as a performance check based on it.

Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-read-cache.c

@@ -0,0 +1,13 @@
+#include "cache.h"
+
+int main (int argc, char **argv)
+{
+	int i, cnt = 1;
+	if (argc == 2)
+		cnt = strtol(argv[1], NULL, 0);
+	for (i = 0; i < cnt; i++) {
+		read_cache();
+		discard_cache();
+	}
+	return 0;
+}