[TW-1702] On-modify hook script only partially modifies other task

_Robin Green on 2015-09-19T18:43:02Z says:_

I have written an on-modify hook which ask the user what to do if due dates are inconsistent with due dates of dependencies, and if the user so requests, either tries to modify the due dates of the current task, or of the dependencies.

This hook seems to work when the user opts to modify the current task, but after I launch `task modify` from the hook to modify a dependency, the change history in `task info` shows the due date as having been changed, but the due date shown in the output of the very same `task info` command shows that the due date has not in fact been changed, contrary to the change history.