Ensure return value from xread() is always stored into an ssize_t

This patch fixes all calls to xread() where the return value is not
stored into an ssize_t. The patch should not have any effect whatsoever,
other than putting better/more appropriate type names on variables.

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

builtin-unpack-objects.c

@@ -34,7 +34,7 @@ static void *fill(int min)
 		offset = 0;
 	}
 	do {
-		int ret = xread(0, buffer + len, sizeof(buffer) - len);
+		ssize_t ret = xread(0, buffer + len, sizeof(buffer) - len);
 		if (ret <= 0) {
 			if (!ret)
 				die("early EOF");