Merging an MR with no changes, but actual commits isn't marked as merged

Summary

In https://gitlab.kitware.com/cmake/cmake/merge_requests/1257 (running Gitlab 9.5.4 gitlab-ce@fbffc274b5df7e79222aeaf55bf5b3b9a466f4cc), an MR which has an empty diff, but actual commits was merged (externally) properly, but not marked as Merged.

Steps to reproduce

Create an MR with no diffs (a merge of another branch using -s ours, a commit and revert, etc.). Merging the MR externally doesn't mark the commit as Merged (unsure if done via the button).

Example Project

I'll try and get to it later today.

What is the current bug behavior?

Merged commits don't mark the MR as merged.

What is the expected correct behavior?

The MR should be in the Merged state.