This should produce better results at squash/split operations. Since "op diff"
targets can be flipped, this patch implements basic handling of reversed
predecessors graph. It should also work for sibling operations so long as there
are no multiple greatest common ancestors.
resolve_transitive_edges() takes additional "start" parameter. It might be
useful in order to omit transitive edges in evolution log. Since
walk_predecessors() usually tracks a single commit, it would be wasteful to
build a fully-resolved predecessors graph.
The output looks better if the graph had long parallel history. "--limit=N" is
applied after sorting for consistency with "jj log". The doc also mentions that.
Since TopoGroupedGraphIterator emits predecessors in reverse order at squash
point, we no longer need to tweak the visiting order by walk_predecessors().
This is slower than traversing commit.predecessor_ids, but allows us to show
associated operations alongside the commits. Since operations are (mostly)
traversed in chronological order, the evolution graph is also emitted in that
order.
