Improve DEPM when change tracking is on

Improve DEPM when change tracking is on

A typical problem when change tracking is on is:
1/ insert a new space between two words (as if you want to insert a new one)
2/ move the cursor
3/ see how one has one inserted and one deleted space, whereas one would expect nothing.

The problem is fixed in a general way in master, but here we propose
a simpler one, where the spaces that we keep are the ones at the end
of the sequence of spaces.

Part of #11412.