When a merge request is redirected to a new project, Rails attempts to
construct the proper URL in `url_for` by finding the first route that
matches the given parameters:

1. `controller` -> `projects/merge_requests`
2. `action` -> `show`
3. `namespace_id`
4. `project_id`
5. `id`

In this example, `/*namespace_id/:project_id/commits/*id` matched
instead of `/*namespace_id/:project_id/merge_requests/:id`.

This happens because the Rails `resources` block defines all the
standard #index, #show, #update, etc. routes after the block runs.

We fixed this issue by explicitly specifying the merge request #show
as the first route.

This is similar to https://gitlab.com/gitlab-org/gitlab/issues/31357,
except the solution is a bit simpler because the backend appears to set
a `tab` query string, which allows the redirected route to end up in the
right place. For example, visiting the old `/merge_requests/:id/commits`
appears to cause a redirection to `/merge_requests/:id/?tab=commits`,
which does the right thing.