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: Ib4000e955dec43ba976d1486e1e80d7b64197c80
Reviewed-on: https://gerrit.libreoffice.org/7708
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>