revision: introduce setup_revision_opt

revision: introduce setup_revision_opt

So far the last parameter to setup_revisions() was to specify the default
ref when the command line did not give any (typically "HEAD").  This changes
it to take a pointer to a structure so that we can add other information without
touching too many codepaths in later patches.

There is no functionality change.

Signed-off-by: Junio C Hamano <gitster@pobox.com>