notes-merge: use ssize_t for write_in_full() return value

We store the return value of write_in_full() in a long,
though the return is actually an ssize_t. This probably
doesn't matter much in practice (since the buffer size is
alredy an unsigned long), but it might if the size if
between what can be represented in "long" and "unsigned
long", and if your size_t is larger than a "long" (as it is
on 64-bit Windows).

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes-merge.c

@@ -302,7 +302,7 @@ static void write_buf_to_worktree(const struct object_id *obj,
 	fd = xopen(path, O_WRONLY | O_EXCL | O_CREAT, 0666);
 
 	while (size > 0) {
-		long ret = write_in_full(fd, buf, size);
+		ssize_t ret = write_in_full(fd, buf, size);
 		if (ret < 0) {
 			/* Ignore epipe */
 			if (errno == EPIPE)