sha1_file: use the correct type (ssize_t, not size_t) for read-style function

Using an unsigned type, we would fail to detect a read error and then
proceed to try to write (size_t)-1 bytes.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>