Is your feature request related to a problem? Please describe.
I'm coming from mercurial with "well-described commits" as a workflow (as opposed to "many fixup commits"), where a typical workflow was:

1. write and commit change
2. get review feedback, make changes
3. hg amend -e to include the additional changes, and edit the commit message to address issues with the commit message.

Doing the same thing in jj, for step 3 I have to do jj squash && jj describe @- or something like that. This is annoying.

Describe the solution you'd like
jj squash -e to force opening the editor even if the source commit for squash has no description.

Describe alternatives you've considered
As above:

• jj squash && jj describe @-

or even

• jj describe -m "haha" && jj squash

All of them feel like too much typing for this flow, which is common for me. I could create an alias, but "force invoking the commit message editor" seems like a reasonable thing to want a flag for, to be supported out of the box? (Happy to be told there's a jj philosophical reason that this isn't something with a desire for support, but I couldn't see a ticket on file (even closed) for this.)

Additional context
Potentially #3947, and this is the inverse of #7515.