fdo#76132: Write test for this first.

The crash is a result of the pasted formula cell (single cell) being marked
"shared", which should never happen.

Change-Id: I7b7fa536c520c76b56cce78457e11b6e2eaee76f
fdo#76132: Don't create a formula group of length 1.

That's asking for trouble.

Change-Id: I6aaa4d1e2aed4f166ee216c3eb02d3b7794c9a8c