Merge requests don't add a "what changed" comment upon reopening

Summary

A merge request is closed (due to inactivity or whatever). New code is then pushed and the MR reopened. There is no comment with the "changes since the last update" link.

Steps to reproduce

• Open a merge request
• Close the merge request
• Push code to the source branch
• Reopen the merge request

Example Project

So I realized after making this, I had opened it on here. See !16657 (closed).

What is the current bug behavior?

No comment about the changes since the last push.

What is the expected correct behavior?

A comment with the changes since the MR was last open.