handle_revision_arg: hoist ".." check out of range parsing

handle_revision_arg: hoist ".." check out of range parsing

Since 003c84f6d (specifying ranges: we did not mean to make
".." an empty set, 2011-05-02), we treat the argument ".."
specially. We detect it by noticing that both sides of the
range are empty, and that this is a non-symmetric two-dot
range.

While correct, this makes the code overly complicated. We
can just detect ".." up front before we try to do further
parsing. This avoids having to de-munge the NUL from dotdot,
and lets us eliminate an extra const array (which we needed
only to do direct pointer comparisons).

It also removes the one code path from the range-parsing
conditional that requires us to return -1. That will make it
simpler to pull the dotdot parsing out into its own
function.

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