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>