merge commit ID omitted on implicitly "merged" merge requests

Summary

A merge request that only contains commit that have been merged (together with more commits) though another concurrent merge request is shown as "merged" (which seems ok to me), but the "were merged into master with" information is missing the revision ID of the merge commit.

A list of all cases where a merged request can have no merge_commit_sha.

• When importer is used
• When fast-forward merge is used
• When branch is implicitly merged as another branch with same SHA has been merged.

Steps to reproduce

1. create a merge request (MR A)
2. create a new branch based on the branch to be merged by MR A
3. without having merged MR A, create new merge request (MR B) based on that new branch and with the same target branch on the same target repo
4. merge MR B

Example Project

openschoolmaps/OpenSchoolMaps.ch!3 (commits were merged with openschoolmaps/OpenSchoolMaps.ch!12)

What is the current bug behavior?

• MR A is shown as merged
• MR B is shown as merged
• MR A says "The changes were merged into master with 2e272a8f"
• MR B says "The changes were merged into master with" (sentence cut off there)

What is the expected correct behavior?

• MR A is shown as merged
• MR B is shown as merged (I guess)
• MR A says "The changes were merged into master with 2e272a8f"
• **MR B says "The changes were merged into master with 2e272a8f"**
  or
  **MR B says "The changes were merged into master by openschoolmaps/OpenSchoolMaps.ch!12"**
  or just
  **MR B says "The changes were merged into master by another merge request"**
  or even just
  MR B says "The changes were merged into master
  

Relevant logs and/or screenshots

openschoolmaps/OpenSchoolMaps.ch!3 :

openschoolmaps/OpenSchoolMaps.ch!12 :

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

n/a

(How do I get these for gitlab.com?)

Results of GitLab application Check

n/a

(How do I get these for gitlab.com?)

Possible fixes

(I didn't investigate any.)