These should take const buffers as input data, but zlib's
next_in pointer is not const-correct. Let's fix it at the
zlib level, though, so the cast happens in one obvious
place. This should be safe, as a similar cast is used in
zlib's example code for a const array.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>