`ratchetFrom 'origin/main'` can be broken in PR if the PR repo has `main` somewhere different than upstream

Nessuno ha ancora preso questa issue.

https://github.com/diffplug/goomph/pull/191 is an interesting PR. spotless is failing, but it passes if you checkout the branch locally and clone.

The reason for the discrepancy is that `diffplug/goomph` had main at https://github.com/diffplug/goomph/commit/22f4e8d6c22366c4d9b041bec93cc42e1862d9e9, whereas `jmini/goomph` had main at https://github.com/jmini/goomph/commit/4c8da613a16f98155ee167239ced96a335ec1016. Because the PR came from `jmini`, from the PR point of view `origin/main` is `jmini/main`, whereas for most people who checkout the project, `origin/main` is `diffplug/main`.

The workaround is for the PR author to move their `main` to match the upstream project.

Ideally users wouldn't have to deal with this, maybe something like `ratchetFrom 'https://github.com/diffplug/spotless@main'`, and then Spotless can detect the correct remote or add it if missing.

Start with the ratchetFrom 'origin/main' configuration and reproduce the difference between a local checkout and a pull-request repository, using the linked PR as the example. Done should allow a repository-independent reference such as 'https://github.com/diffplug/spotless@main', with the correct remote detected or added when needed.

Scritto dal modello di indicizzazione a partire dal testo della issue.