Reopened MRs do not get "new code pushed" events

Summary

Reopened MRs do not post "source branch updated" statuses to the right places (webhooks and associated MRs). Seen on 8.17 and 9.0; 8.16 was OK.

Steps to reproduce

• Close an MR;
• Reopen the MR;
• Push new code to the source branch

What is the current bug behavior?

No indication that anything has changed.

What is the expected correct behavior?

Webhooks and system comments should be created.