Fix signedness on return value from xread()

The return value from xread() is ssize_t.
Paolo Teti <paolo.teti@gmail.com> pointed out that in this case, the
signed return value was assigned to an unsigned type (size_t). This patch
fixes that.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

pack-write.c

@@ -25,7 +25,7 @@ void fixup_pack_header_footer(int pack_fd,
 
 	buf = xmalloc(buf_sz);
 	for (;;) {
-		size_t n = xread(pack_fd, buf, buf_sz);
+		ssize_t n = xread(pack_fd, buf, buf_sz);
 		if (!n)
 			break;
 		if (n < 0)