Bug #1270

- The 'undo' command is now properly removing backlog entries.
Unit Tests

- Added more undo tests, to cover both cases for backlog rollback.
Documentation

- #1279 Assorted corrections to the task-ref.pdf document (thanks to Benjamin
  Weber).
Merge branch '2.3.0' of tasktools.org:task into 2.3.0

Conflicts:
	ChangeLog
Bug #1356

- #1356 Command reference now mentions /from/to/g.
Bug #1352

- Added reference to #1352 in the NEWS file.
Merge branch '2.3.0' of tasktools.org:task into 2.3.0

Conflicts:
	ChangeLog
Feature #1297

- #1297 The task₋sync(5) man pages is rewritten with examples.