Correct error message for branch creation service

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/216545

Currently we display a misleading error message when a branch cannot
be created because of the incorrect reference.

I change the error message to return a reference name rather than a new
branch name.

app/services/branches/create_service.rb

@@ -14,7 +14,7 @@ module Branches
       if new_branch
         success(new_branch)
       else
-        error("Invalid reference name: #{branch_name}")
+        error("Invalid reference name: #{ref}")
       end
     rescue Gitlab::Git::PreReceiveError => ex
       error(ex.message)

spec/requests/api/branches_spec.rb

@@ -623,7 +623,7 @@ describe API::Branches do
       post api(route, user), params: { branch: 'new_design3', ref: 'foo' }
 
       expect(response).to have_gitlab_http_status(:bad_request)
-      expect(json_response['message']).to eq('Invalid reference name: new_design3')
+      expect(json_response['message']).to eq('Invalid reference name: foo')
     end
   end
 

spec/requests/api/graphql/mutations/branches/create_spec.rb

@@ -38,7 +38,7 @@ describe 'Creation of a new branch' do
       let(:ref) { 'unknown' }
 
       it_behaves_like 'a mutation that returns errors in the response',
-                      errors: ['Invalid reference name: new_branch']
+                      errors: ['Invalid reference name: unknown']
     end
   end
 end

spec/services/branches/create_service_spec.rb

@@ -24,18 +24,29 @@ describe Branches::CreateService do
       end
     end
 
-    context 'when creating a branch fails' do
+    context 'when branch already exists' do
+      let(:project) { create(:project_empty_repo) }
+
+      it 'returns an error' do
+        result = service.execute('master', 'master')
+
+        expect(result[:status]).to eq(:error)
+        expect(result[:message]).to eq('Branch already exists')
+      end
+    end
+
+    context 'when incorrect reference is provided' do
       let(:project) { create(:project_empty_repo) }
 
       before do
         allow(project.repository).to receive(:add_branch).and_return(false)
       end
 
-      it 'returns an error with the branch name' do
-        result = service.execute('my-feature', 'master')
+      it 'returns an error with reference name' do
+        result = service.execute('my-feature', 'unknown')
 
         expect(result[:status]).to eq(:error)
-        expect(result[:message]).to eq("Invalid reference name: my-feature")
+        expect(result[:message]).to eq('Invalid reference name: unknown')
       end
     end
   end