Fix conflicting MR link when MR created from fork

What does this MR do and why?

This MR fixes conflicting MR link when MR is created from forked project

Screenshots or screen recordings

Before	After
before	after

How to set up and validate locally

1. Fork project
2. In forked project: create new MR where source branch is from forked project & target branch is from original project
3. In forked project: try to open new MR with same branches as in step 2
4. Click "Compare branches and continue"

Error should appear at the top with valid link to conflicting MR

Related to #426138 (closed)