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
(cherry picked from commit f571104ef38ba9f7f6073e22c2374add7aa73887)
Reviewed-on: https://gerrit.libreoffice.org/7707
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>