[Bug]: GitLab merge requests don't link #1, !5, commits or mentions #30522

Area

Pull requests (GitLab provider)

Steps to reproduce

1. Open a GitLab merge request whose description mentions other issues, merge requests, commits or people. I used !10 on TonybynMp4/git-ansible, whose description says "closes oxc stack #1, follows up on !5, related to TonybynMp4/git-ansible-fork@dac72fa1 and dac72fa1…, cc @TonybynMp4".
2. Look at the Description on the Summary tab.

Expected behavior

The references are links, as they are on GitLab and on GitHub pull requests in T3. !5 opens merge request !5 inside T3 and shows its preview card on hover.

Actual behavior

All of it is plain text. The GitLab source control definition has no referenceAutolinks, so the web and mobile markdown renderers link nothing for GitLab. A test in client-runtime (pullRequestMarkdown.test.ts) even pins that GitLab links nothing, from when mobile applied GitHub's grammar to every host.

Impact

Following a reference means copying it and searching GitLab. !N is the most common way GitLab users point at another merge request.

Version or commit

9aa80a0dc4 (main)

Environment

Linux, web dev build, gitlab.com.

Screenshots, recordings, or supporting files

On main, the references in !10's description are plain text:

Workaround

Open the merge request on GitLab.