Add support for patch link extension for commit links on GFM

What does this MR do?

Solves #17939 (closed) by correctly processing commit links with patch extension. This MR also leaves the groundwork to apply other extensions to commits or other reference filters (.diff) for instance.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #17939 (closed)