[Feature request]: Opt-in gitignore-style cleanup mode #6815

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

Would there be interest in adding an opt-in gitignore-style mode for flatpak-builder's cleanup patterns?

The current syntax only supports positive removal patterns, so it is not possible to remove everything under a directory while keeping one specific file. For example:

cleanup-mode: gitignore

cleanup:
  - /bin/*
  - '!/bin/avahi-browse'

The proposed mode would preserve the current behavior by default and support ordered rules, negation, escaping, directory matching, character classes, and ** patterns.

This came up while packaging Kodi, where avahi-browse must be retained while other Avahi binaries are removed. Would this feature be useful more broadly, and would the proposed opt-in syntax be acceptable?

flathub/tv.kodi.Kodi#784 (comment)