FR: Upload to Gerrit without overwriting the commit message

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

1. jj gerrit upload always overwrites the remote commit message (which I often need to modify to add extra footers required by Gerrit).

So you could argue I should not modify the commit message in Gerrit, but

2. jj gerrit upload is rejected if the only change is an update of the commit message.

Describe the solution you'd like

Best solution:

• If the commit message did not change locally, do not overwrite the remote commit message.
• If it was modified locally, do overwrite the remote commit message
• Add--update-commit-message/--do-not-update-commit-message arguments to force the behavior.

Second best would be to simply never update the commit message, unless --update-commit-message is added.

Third best is a --do-not-update-commit-message argument (or something shorter ;-)).