read-cache: free cache in discard_index

discard_cache doesn't have to free the array of cache entries, because the next call of read_cache can simply reuse it, as they all operate on the global variable the_index. discard_index on the other hand does have to free it, because it can be used e.g. with index_state variables on the stack, in which case a missing free would cause an unrecoverable leak. This patch releases the memory and removes a comment that was relevant for discard_cache but has become outdated. Since discard_cache is just a wrapper around discard_index nowadays, we lose the optimization that avoids reallocation of that array within loops of read_cache and discard_cache. That doesn't cause a performance regression for me, however (HEAD = this patch, HEAD^ = master + p0002): Test // HEAD^ HEAD ---------------\\----------------------------------------------------- 0002.1: read_ca// 1000 times 0.62(0.58+0.04) 0.61(0.58+0.02) -1.6% Suggested-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>