The XLCells has been fixed to match original behavior - each cell is
considered an area and outside border should change individual cell
border. Thus we have to do it same way to keep compatibility, although
it's kind of insane.

The former code needed to interpret each cell as a separate area
for outside/inside borders, but that was incredibly inefficient.
It didn't leverage bulk operations. Add a flag to make a special path
for cells in API objects. Not great, but better than horrible
performance or breaking change.