Merge request shows up as mergable when it isn't

Summary

Sometimes, the Merge button will be clickable even though the merge will fail.

Steps to reproduce

Have a MR that changes a file that is also changed in the target branch.

What is the current bug behavior?

When clicking the Merge button, merging fails with a message like:

Something went wrong during merge: Could not merge SHAa onto SHAb: n files updated, m files merged, l files removed, p files unresolved use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon . Refreshing in x seconds to show the updated status... 

What is the expected correct behavior?

It should tell me that the merge is not possible, or allow the merge by using a better merge tool (internally, Heptapod uses the "internal:fail" hg merge tool that is unreasonably strict).

My gut feeling is that the check for merging is still the GitLab one and hasn't been adapted to Mercurial.