service=gitlab invokes Gitea's GitLab-API-based downloader (for issues/
PRs/releases metadata), which calls the source's /api/v4/projects/...
REST API rather than doing a plain git clone. That was 404ing against
home GitLab and getting swallowed into a generic 'InternalServerError:
404 Not Found' with no further detail. We only want a plain pull-mirror
of the git repo itself, so service=git forces the plain git-clone
downloader instead.