Gitlab integration: 'Diff' button generates incorrect URL (mantis & gitlab domains concatenated)

Created by: seanm

Not sure if I've configured something wrong, or it's a bug, but the 'Diff' button in every changeset has a URL of this form:

https://mantis.example.com/bugs/gitlab.example.com/namespace/project/commit/SHA

when it should be:

https://gitlab.example.com/namespace/project/commit/SHA

Note how the mantis URL (https://mantis.example.com/bugs/) is prepended.

I've looked through the docs and settings, but I don't see anything that explains this... so perhaps it's a bug...