Add failing test for Issue #1266.
When pushing onto the undo stack, elide zero-length entries

A redo that consists of a tree of three or more linked
commands (like 'cw) can create a zero-length
'UndoRedoData.Linked' entry in the undo stack.  Allow
this to happen but correct for it when pushing the
next normal entry.

Closes #1266
Merge branch 'fixes-1266'
Add failing unit tests for issue #1269

- Add test for word forward from entirely blank line
- Add test for word forward from last word of last line
- Fix order of expected vs. actual in related tests