fix: disallow blame on directories (#6716)
- Don't allow the blame operation on directories.
- Added integration test.
- Resolves forgejo/forgejo#6533
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6716
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>