Detect that a version or sequence number is "taken" in main

[kaaveland]

Here's an annoying scenario that happens in every project I've worked in that uses flyway: Bob and Alice branch from main at the same time, and both start working on a database migration. They end up integrating their branch more or less simultaneously, causing a conflict where they've both "claimed" the same version. Since eugene is already git-aware, it should be able to pick this up in CI/CD, if one of the branches is already merged by the time CI for the other one runs.

Hard to see a way to pick this up if Bob and Alice push their branches / open pull requests at the same time, and let them linger.