Handle force_remove_source_branch when creating merge request

Creating a merge request with
`merge_request[force_remove_source_branch]` parameter would result in an
Error 500 since this attribute was passed directly to the merge request.
Fix this by properly parsing this attribute into `merge_params`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51220

app/services/merge_requests/build_service.rb

@@ -10,6 +10,7 @@ module MergeRequests
       # TODO: this should handle all quick actions that don't have side effects
       # https://gitlab.com/gitlab-org/gitlab-ce/issues/53658
       merge_quick_actions_into_params!(merge_request, only: [:target_branch])
+      merge_request.merge_params['force_remove_source_branch'] = params.delete(:force_remove_source_branch) if params.has_key?(:force_remove_source_branch)
       merge_request.assign_attributes(params)
 
       merge_request.author = current_user

changelogs/unreleased/sh-fix-issue-51220.yml

+---
+title: Handle force_remove_source_branch when creating merge request
+merge_request: 23281
+author:
+type: fixed

spec/services/merge_requests/build_service_spec.rb

@@ -21,15 +21,20 @@ describe MergeRequests::BuildService do
   let(:commit_2) { double(:commit_2, sha: 'f00ba7', safe_message: 'This is a bad commit message!') }
   let(:commits) { nil }
 
-  let(:service) do
-    described_class.new(project, user,
+  let(:params) do
+    {
       description: description,
       source_branch: source_branch,
       target_branch: target_branch,
       source_project: source_project,
       target_project: target_project,
       milestone_id: milestone_id,
-                                    label_ids: label_ids)
+      label_ids: label_ids
+    }
+  end
+
+  let(:service) do
+    described_class.new(project, user, params)
   end
 
   before do
@@ -56,6 +61,19 @@ describe MergeRequests::BuildService do
       merge_request
     end
 
+    it 'does not assign force_remove_source_branch' do
+      expect(merge_request.force_remove_source_branch?).to be_falsey
+    end
+
+    context 'with force_remove_source_branch parameter' do
+      let(:mr_params) { params.merge(force_remove_source_branch: '1') }
+      let(:merge_request) { described_class.new(project, user, mr_params).execute }
+
+      it 'assigns force_remove_source_branch' do
+        expect(merge_request.force_remove_source_branch?).to be_truthy
+      end
+    end
+
     context 'missing source branch' do
       let(:source_branch) { '' }