message: Fix sinon fake to return a promise like the method it replaces
  This was causing an error in tests that whilst not causing the tests to fail,
  was annoying.
diff: @@ -848,8 +848,8 @@ describe('git/writer/hyperledger', function() {
  await destroyDefaultEnvironment(env);
  });
  - it('writes to hyperledger if configured when writing', async function () {
  - let fakePush = fake();
  + it('writes to hyperledger if configured when writing', async function () { let
  + fakePush = fake.returns(new Promise(resolve => resolve()));
  replace(gitChain, 'push', fakePush);
  