log: properly render elided roots in reversed mode

[CGamesPlay]

This is my first contribution to jj. I experimented with several approaches to this problem and this one is the best approach I could come up with.

This properly addresses #9149, which was incorrectly marked as a duplicate of #1092. Although the two issues are related, this does NOT resolve the latter but does fix the correctness issue shown in the former.

Related issues:

• Properly resolves jj log --reversed shows misleading commit history #9149
• Resolves FR: make excluded commits more compact in jj log #5937 by no longer using Ancestor::Anonymous nodes
• Related but unfixed FR: improve the rendering of jj log --reversed with sapling #1092 because this is a limitation of sapling renderdag.

Notable changes:

• the vertical line now always goes to the end of a multi-line node before showing the ~
• the extra blank lines that were forced around ~ lines disappear, but disconnected components ALWAYS get a blank line separating them
• merges where multiple parents are elided now show multiple ~ nodes (the number of parents is preserved)
• the final root ~ now respects ui.log-synthetic-elided-nodes, and is rendered using the log_node template

To address potential concerns:

• Why are evolog and op log not affected? These logs never produce disconnected components. It might be nice to use these synthetic elided nodes so that users can customize the log_node template, but not critical to correctness.
• Why not remove the concept of a "missing edge" entirely? That's a hugely invasive change, and doesn't seem worthwhile.
• Why not add proper support for reversed graphs to sapling's renderdag? This is the only approach I can see that will actually resolve the differences shown in FR: improve the rendering of jj log --reversed with sapling #1092, but it's complicated. We walk commits from leafs to root because that's how the database is set up, and this assumption is baked into renderdag, so it can perform online rendering of the graph in O(N) space.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[CGamesPlay]

(differences between stable and nightly rustfmt are driving me crazy)