FR: jj undo should print what operation is being undone

[rdamazio]

Is your feature request related to a problem? Please describe.
When you run jj undo, it prints the new commit and parent, but doesn't tell me what was undone.
This requires going back and forth between undo and op log which is inconvenient.

Describe the solution you'd like
It should print what operation is being undone (e.g. "workspace snapshot triggered by jj st", "jj rebase", "workspace snapshot + jj rebase", etc.)

Describe alternatives you've considered
Running jj op log and keeping the result open to understand what's happening, re-running it periodically if there's many operations to undo.

[PhilipMetzger]

this also belongs to #3700, as printing jj op undo twice is useless.