Extend "Push and merge when pipeline succeeds" command line feature with `-o merge_request.source_branch=`

Extending number of -o merge_request options with source_branch for generating merge request

git push \
  -o merge_request.create \
  -o merge_request.target_source=dev \
  -o merge_request.target_branch=stage \
  -o merge_request.merge_when_pipeline_succeeds

Current help: https://docs.gitlab.com/ee/user/project/merge_requests/#create-a-new-merge-request-using-git-push-options