[TW-103] Undo changes of recurring tasks in one step

_Johannes Schlatow on 2011-04-01T10:57:27Z says:_

E.g. completing a recurring task adds two entries to the undo.data file, because there are two steps:

* the child tasks status is changed
* the parents task mask is modified

Thus, in order to undo the changes made with 'task done' I have to execute 'task undo' twice. I guess for the most users this is very confusing. Moreover I assume this could lead to inconsistent states if I solely reverse the modification of the parent task.

It should be possible to tweak the undo command so that it recognizes atomic operations on recurring tasks and reverses them at once.