FR: Render elided commits in jj log as synthetic node

[martinvonz]

Is your feature request related to a problem? Please describe.

We currently render elided nodes in the log output as dashed lines, like this:

◉  tou yuya@tcha.org 2 days ago 99f0e7f8
╷  cli: remove workaround for ": {source}" embedded in error message
╷ ◉  oxlm ben@brittain.org 2 days ago pager@origin cb232258
╭─╯  cli: Provide better flags for the "less" pager than default
◉  kksnw ben@brittain.org 2 days ago ae3404cd
│
~

It's not obvious what the dashed lines mean.

Describe the solution you'd like

It would be clearer if we inserted a synthetic node where we elide commits:

◉  tou yuya@tcha.org 2 days ago 99f0e7f8
╷  cli: remove workaround for ": {source}" embedded in error message
◌ (elided commits)
╷
╷ ◉  oxlm ben@brittain.org 2 days ago pager@origin cb232258
╭─╯  cli: Provide better flags for the "less" pager than default
◉  kksnw ben@brittain.org 2 days ago ae3404cd
│
~

We might not even need to draw the dashed lines if we do (but they're not expensive or anything, so we could).

If we draw a synthetic node for elided commits in the middle of the graph, then it would also make sense to also do that instead of the tilde we currently draw for elided nodes going to the root.

We may or may not want to also mention the number of elided commits (e.g. "(1234 elided commits)"). Depends on how cheap we can make it.

This would also solve #1252.

Describe alternatives you've considered

We could instead use some clearer symbol for dashed line, possibly using more space.

[epage]

For git-stack, I was considering a vertical ellipses for the synthetic node for elided commits.

[ilyagr]

I actually think ~ is pretty good for elided commits.

[arxanas]

Here's an example of what omitted commits look like in git-branchless:

Note that the log is reversed and trunk commits are represented with an ellipsis without a commit count. (Also, it doesn't use ◌, but it should because that's clearly a better choice of icon.)

[thoughtpolice]

See also: #3124 for an experiment I'm running with a totally different icon which I think looks like a "scissor snip."

[bnjmnt4n]

Closing since this was done in #3027. #3099 tracks the display of number of elided commits in the synthetic node.