remote graphs doesn't work for non-default repository branch

Issue

We create workload branch from project's default branch. If we start a workflow in non-default branch the workload branch can't access files from other branch.

Steps to reproduce

• Create a Jenkinsfile in any branch.
• Start the workflow using Convert to GitLab CI button.
• Checkout WF execution. It should error out with file not found

Implementation Plan

Pass the git branch option in the api call to start workflow.