fdo#69244: Write unit test for this.

Change-Id: I5bfc8ad1566dbfc2813aadc8f08ba1eccca608ca
fdo#69244: Avoid putting these cells in formula tree prematurely.

Marking the cell dirty alone appears to be sufficient to trigger resetting
of number format.  SetDirty() would mark it dirty *and* put the cell into
formula tree, which would prevent proper value propagation as seen in the
bug report.

Change-Id: Ie68f996112938fe286a9bd50c38404f9df6f4ca1