Allow for a directory `default_merge_message` in the `CustomPath` to be used as a fallback if no merge template is specified by the repository. Effectively being a global merge message template feature.
Resolves #6648
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8347
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: kochklops <kochklops@noreply.codeberg.org>
Co-committed-by: kochklops <kochklops@noreply.codeberg.org>
