revisions: split handle_revision_opt() from setup_revisions()

Add two fields to struct rev_info:

 - .def to store --default argument; and
 - .show_merge 1-bit field.

handle_revision_opt() is able to deal with any revision option, and
consumes them, and leaves revision arguments or pseudo arguments
(like --all, --not, ...) in place.

For now setup_revisions() does a pass of handle_revision_opt() again
so that code not using it in a parse-opt parser still work the same.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>