Fix position of cursor when DEPM is used in change tracking mode
When change tracking is active, it may happen that spaces are marked
as deleted instead of being removed. Therefore it is better to measure
the change of size of the paragraph to know how to offset the cursor.
Moreover, it the cursor was strictly after the start of the sequence
of spaces, we ensure that it is still the case.
This commit is _not_ a backport from master, which has diverged.