This allows syncing a branch of a fork with a branch of the base repo. It looks like this:

This is only possible, if the fork don't have commits that are not in the main repo.
The feature is already working, but it is missing Finetuning, a better API, translations and tests, so this is currently WIP. It is also not tested with go-git.
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## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
- [PR](https://codeberg.org/forgejo/forgejo/pulls/2364): <!--number 2364 --><!--line 0 --><!--description c3luYyBmb3Jrcw==-->sync forks<!--description-->
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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/2364
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: JakobDev <jakobdev@gmx.de>
Co-committed-by: JakobDev <jakobdev@gmx.de>