All Files in MergeRequestsController#show should use the same `route-map.yml`

Currently, the route-map.yml file used to calculate the external route in MergeRequestsController#show is fetched per diff_file.content_sha.

If I understand the feature correctly, the route-map.yml used should match the commit sha of the most recently deployed application, or possibly HEAD for the merge request branch, but not change per file.

This bug has a downstream performance affect in that multiple fetches of route-map.yml for different Commit SHAs in a merge request are contributing to a Gitaly n+1.

cc @DouweM for more information on route-map.yml