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FakeVim: Use QTextDocument::revision() for undo/redo

Using QTextDocument::availableUndoSteps() to get revision of document is
very bad because it undo/redo stack starts to break when maximal number
of undo is reached.

On the other hand QTextDocument::revision() number is always increased,
even on QTextDocument::redo(), so external undo/redo can still break
undo/redo commands in FakeVim.

Change-Id: If1698df8f43a878295eebddd59aebe304cdf3081
Reviewed-by: hjk <hjk121@nokiamail.com>