graphlog: display obsolete changeset as "x" Changeset detected as obsolete will be displayed as "x" instead of 'o': o new rewritten changeset | | x old obsolete changeset |/ | o base This will be useful even when some obsolete changeset will be "hidden" because not all obsolete changeset can be hidden. If an obsolete changeset have non-obsolete descendant we can't simply hide it. And having a clear visual hint that the changeset is obsolete is useful. The main reason to make this minor change right now is to: 1) introduce an officiel user of the `ctx.obsolete()` method that will detect breakage earlier than third party code (mutable, hgview) 2) Do not display any vocabulary related to obsolete. Such vocabulary will require discussion.