And do it once when importing xls, xlsx, and ods documents.

Although xls(x) file formats support shared formula natively, it's
still beneficial to re-group it as some old xls documents limit the
length of shared formula span to only 64, or some don't use shared
formulas at all even though they could. And re-grouping just once
is not terribly expensive.

Change-Id: Iff9c605d19baa187553ddab6af8b9fbd4c366d7d