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

sha1_file.c

@@ -2733,7 +2733,7 @@ static int index_stream(unsigned char *sha1, int fd, size_t size,
 	while (size) {
 		char buf[10240];
 		size_t sz = size < sizeof(buf) ? size : sizeof(buf);
-		size_t actual;
+		ssize_t actual;
 
 		actual = read_in_full(fd, buf, sz);
 		if (actual < 0)