Autocomplete doesn't work on diff notes

Summary

Users and Issues autocomplete doesn't work on diff notes on project MRs.

Steps to reproduce

• Go to any MR
• Switch to changes tab
• Click on callout icon to the left of diff to add a diff note
• Use @ or # symbol in Note form to see context menu for autocomplete

What is the current bug behavior?

Autocomplete for users (@) or issues (#) doesn't show up.

What is the expected correct behavior?

Context menu should show up.