ux: it's often unclear what the destination commit for each change is

[arxanas]

Description of the bug

Particularly in jj, people often don't know when using e.g. jj squash whether the selected changes will go in the parent or child commit.

It's partially addressed by

• Use unicode bullets for tristate check symbols #46
• using the "commit message" editor API as it currently exists to be able to label one of the commits.

Expected behavior

• It should be possible to meaningfully identify both sides of the commit/split.
• One proposal is to add a caller-specified help label, like "select changes that will go into the parent/child commit".
• In the long-term, supporting multi-way splits with a good interface would probably incidentally resolve the problem.

Actual behavior

No response

Version of rustc

No response