Code searches allow you to choose branch name or tag when searching for
code. Previously the blob search results were linking to the URL with
the SHA of the branch and not the actual branch name being searched.
This works OK in some cases but usually the user expecting to be linked
to `/master/` instead of `/<some-sha>/` and in particular it is
problematic because when you are loading a file in a specific SHA you
cannot click the edit button to edit that file you just loaded.

We fix this by using the param `repository_ref` when present and
defaulting to the `default_branch` when absent.

See more at https://gitlab.com/gitlab-org/gitlab/-/issues/254932