1. Enter 18-sep-2010 in A1.
2. Select A1, press F2, press Enter.
---> now we have a string

Initially we recognize the edit string ("18/9/2010") as a date.  This
happens fairly early in wbcg_edit_finish.

Then we carry on with the string and deep down below cmd_set_text_full we
end up re-interpreting the string, but by that time we do not see the format
and fail to recognize the string as a date, let alone the right date.

I would feel happier if we only matched once and used the value from then
on.  That would also avoid thinking about that should happen if we enter
1/2 into a range with different formats.  I don't think we want some cells
to get Feb-1, others Jan-2, and yet other 0.5 although doing that would
not be as bad as what we do now.  (It would be a kind of self-inflicted
nonsense.)