read_gitfile_gently: use ssize_t to hold read result

Otherwise, a negative error return becomes a very large read
value. We catch this in practice because we compare the
expected and actual numbers of bytes (and you are not likely
to be reading (size_t)-1 bytes), but this makes the
correctness a little more obvious.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>