Allow users to describe commits

[icorbrey]

Description

I want a way to change the description on a commit from within jjj. Ideally, I should be able to e.g. press <d> while a single commit is highlighted to open a text prompt for the description. If the commit already has a description, it should be available for me to modify. Once I'm happy with my description, I should be able to run jj describe -r <change ID> on that commit.

Use Case

Core Jujutsu functionality

Last Steps

• I agree to abide by this project's code of conduct.
• I would like to work on this and submit a PR with my changes.
  (Optional, but contributions are greatly appreciated!)