…-submit`

The idea being to create other "forges", representing Git hosting providers, that can be pushed to in ways that support code review workflows.

The intention is to use this after Phabricator's `arc diff` amends the current commit's message, so that we can adopt the resulting message.

(Only if the branches don't already exist.)

Since we also use the testing infrastructure to submit to Phabricator, it's inaccurate to say "test" in many of these places. (jj also plans to use `jj run` instead of `jj test`.)