Extracted from #12824 on suggestion of @limiting-factor; refactored after @Gusted pointed out forgejo/forgejo!12335.
Behavior change: previously a missing `hooks` folder in a repository tree (should not happen before forgejo/forgejo!12335) would return a 500 on `/api/v1/repos/%s/hooks/git`. It now returns a 200, with the same reply as an empty `hooks` folder.
Test has been added to ensure correct handling of missing `hooks` folder and of its creation if necessary.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12833
Reviewed-by: limiting-factor <limiting-factor@noreply.codeberg.org>
