Check for existing merge requests for a source branch during compare

Problem to solve

Currently, when you create a merge request using /merge_requests/new that compares branches as a prior step, you can continue filling out the form on the new merge request and you will get an error if there's an existing merge request for a source branch upon clicking Submit merge request, which will discard all the form data like merge request title, description, etc.

Before compare	After submit

Proposal

When there's an existing merge request for a source branch, you should get the error upon clicking Compare branches and continue.