handle_revision_arg: stop using "dotdot" as a generic pointer

handle_revision_arg: stop using "dotdot" as a generic pointer

The handle_revision_arg() function has a "dotdot" variable
that it uses to find a ".." or "..." in the argument. If we
don't find one, we look for other marks, like "^!". But we
just keep re-using the "dotdot" variable, which is
confusing.

Let's introduce a separate "mark" variable that can be used
for these other marks. They still reuse the same variable,
but at least the name is no longer actively misleading.

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