Add spec for Project#write_repository_config

spec/models/project_spec.rb

@@ -3155,4 +3155,26 @@ describe Project do
       it { is_expected.to eq(platform_kubernetes) }
     end
   end
+
+  describe '#write_repository_config' do
+    set(:project) { create(:project, :repository) }
+
+    it 'writes full path in .git/config when key is missing' do
+      project.write_repository_config
+
+      expect(project.repo.config['gitlab.fullpath']).to eq project.full_path
+    end
+
+    it 'updates full path in .git/config when key is present' do
+      project.write_repository_config(gl_full_path: 'old/path')
+
+      expect { project.write_repository_config }.to change { project.repo.config['gitlab.fullpath'] }.from('old/path').to(project.full_path)
+    end
+
+    it 'does not raise an error with an empty repository' do
+      project = create(:project_empty_repo)
+
+      expect { project.write_repository_config }.not_to raise_error
+    end
+  end
 end