Allow linking to commits in other repos

### Feature Description

GitHub allows referring to issues in other repos using `owner/repo#issue` (which we support already, just for clarity).
Similarly, it allows referencing commits in other repos using `owner/repo@sha`, i.e. delvh/gitea@51a92cb8218b6702a5a0c8f921eda02456332748 (which Gitea doesn't).
Gitea should support it too.

### Screenshots

_No response_