Use __VA_ARGS__ for all of error's arguments

QNX 6.3.2 uses GCC 2.95.3 by default, and GCC 2.95.3 doesn't remove the
comma if the error macro's variable argument is left out.

Instead of testing for a sufficiently recent version of GCC, make
__VA_ARGS__ match all of the arguments.

Signed-off-by: Matt Kraai <matt.kraai@amo.abbott.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>