revset_graph: queue linear history per branch, emit at fork point

This is similar to Mercurial's "topo" sorting, but merge handling seems
different. Mercurial's dagop.toposort() tracks [(roots, descendants)], and
merged ancestors are combined into a single tracking record. OTOH, ours tracks
{root: descendants}, and merged ancestors are split into separate tracking
records. I don't know which one will produce a better result, but I think
the latter is easier to implement.

Compared to Sapling's beautify_graph(), this is lazy, and can roughly preserve
the index (or chronological) order. I tried beautify_graph() with prioritizing
the @ commit, but the result seemed too aggressively reordered. Perhaps, for
merge-heavy history (or branchy history in reverse order), beautify_graph()
will produce significantly better result. For my wip branches (~30 branches,
a couple of commits per branch), this works pretty well.

Closes jj-vcs#242

CHANGELOG.md

@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### New features
 
+* `jj log` output is now topologically grouped.
+  [#242](https://github.com/martinvonz/jj/issues/242)
+
 ### Fixed bugs
 
 ## [0.8.0] - 2023-07-09