[chat integration] link "compare changes" broken on self-hosted install since 16.0

Since migration to %16.0, the link "compare changes" is broken in slack and google chat
it brings to
https://xxx.xxx.tld/group/project/compare/77bff755be7f4ee788f71b256b88146200735884...cb16035e307e978ba9577dc1f0d184f763f5c460
instead of
https://xxx.xxx.tld/group/project/-/compare/77bff755be7f4ee788f71b256b88146200735884...cb16035e307e978ba9577dc1f0d184f763f5c460 (mind the addition "-/")