The diffs_metadata.json request in StartupJS is not matching the one done from within the Diffs app

Summary

The path being added to the StartupJS for diffs_metadata is not matching the one used by the app.

This causes the request to be made twice instead of levering the benefit of StartupJS.

Steps to reproduce

Open any MR in the Changes tab with the Network tab of Dev Tools open.

Example Project

gitlab-org/quality/triage-ops!680 (diffs)

What is the current bug behavior?

Two diffs_metadata are made.

What is the expected correct behavior?

Only one diffs_metadata is made, very early on in the page lifecycle (StartupJS).

Also, check that it works for both Inline and Parallel views.