feat: Allow the fixup helper to return commits on the main branch with a warning.

[mricherzhagen]

PR Description

Sometimes it is permissible/required to fixup a commit, even though it is already on the main branch. Even if it is not permissible it might be useful information which commit would have to be changed.

This PR changes the ctrl-f function to instead of showing an error when the commit is already merged to only show a confirmation warning.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[mricherzhagen]

This PR now shares a commit with #5201 that moves the creation of the Handler function into a helper function.