feat: Make Fixup Helper ignore fixup-commits, if multiple base commits are found.

[mricherzhagen]

PR Description

Sometimes I run into the "Multiple base commits found" error message, when I'm using the ctrl-f function to find the commit to fixup. But most often than not it lists the base commit I want and a bunch of unsquashed fixup! commits.

This PR makes lazygit check for that case and then offers the only non-fixup commit found with a warning/confirmation dialog.

This PR shares a commit with #5193 that moves the creation of the Handler function into a helper function.

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

[stefanhaller]

This is a very welcome change; I frequently run into this situation too, and find it annoying.

However, the implementation is not quite good enough: you are only looking for any fixup! commits, but it's important that you only consider fixup commits for the same base commit. (This needs a prefix comparison, since the subject of the fixup might be shorter than the original.)

Also, once you detected this situation, I think we should select the base commit without showing a confirmation. I find the confirmation more confusing than helpful actually.