grep: re-order rev-parsing loop

grep: re-order rev-parsing loop

We loop over the arguments, but every branch of the loop
hits either a "continue" or a "break". Surely we can make
this simpler.

The final conditional is:

  if (arg is a rev) {
	  ... handle rev ...
	  continue;
  }
  break;

We can rewrite this as:

  if (arg is not a rev)
	  break;

  ... handle rev ...

That makes the flow a little bit simpler, and will make
things much easier to follow when we add more logic in
future patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>