Switch the source for Diffs suggestion commit message to the MR Notes store

What does this MR do?

For #300749 (closed)

Because the MR Metadata is always loaded when the page first starts up for any MR page, this MR:

1. Switches the authoritative source of MR Metadata for the suggestion commit placeholder to be the "global" mr_notes store instead of the more specific diffs store.

MR	Description
!56575 (merged)	This MR ingests the required endpoint into the MR Notes app.
!56594 (merged)	This MR actually loads the diffs_metadata.json data and places it into the state for the MR Notes app
We're here 👉🏻	This MR switches the Diffs app to use the MR Notes app location to fill the placeholder data
!58136 (merged)	This MR makes changes to enable getting the Diff file information into the Notes app on the Overview tab

Screenshots (strongly suggested)

Note here that this switch to the new data source doesn't change the Diffs output, but does significantly fix the Overview output.
To be clear, this change is not intended to fully fix the Overview output (which is why it still has unhydrated variables).
The fixes in the Overview tab are just a side effect of the update to the Diffs app.

	Before	After
Changes
Overview

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