Do not look up commit again when it is passed to RelativeLinkFilter
## What does this MR do?
Use `context[:commit]` in RelativeLinkFilter instead of looking up commit using `context[:ref]`.
## Why was this MR needed?
Even though the commit object was already passed, unnecessary I/O is done to retrieve the commit object.
## What are the relevant issue numbers?
Fixes#20026
See merge request !5455