[INTERNAL] sap.ui.fl: use FakePromise in isChangeHandlerRevertible
[INTERNAL] sap.ui.fl: use FakePromise in isChangeHandlerRevertible

a previous commit introduced coding that always created a real Promise
even if it was not needed. Now when we don't need a Promise we use a
FakePromise and stay sync

Change-Id: I3b13fb9d9e74ad5edf055f29ca23f502e4bedf6f
Signed-off-by: Kevin Edinger <2cd308f6e534f22a23b481bd971d726fce0a3425@sap.com>

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5