src/vs/workbench/api/common/extHostProgress.ts/mergeProgress
function mergeProgress(result: IProgressStep, currentValue: IProgressStep): IProgressStep {
	result.message = currentValue.message;
	if (typeof currentValue.increment === 'number') {
		if (typeof result.increment === 'number') {
			result.increment += currentValue.increment;
		} else {
			result.increment = currentValue.increment;
		}
	}

	return result;
}