(bug 37980) Cutting all text and repasting it breaks the editor
This was caused by a bug in fixupInsertion that caused it to believe
that inserting something like "a</p><p>b</p><p>c" into the middle of an
empty paragraph was invalid.
This commit fixes the fixupInsertion bug, which fixes the
select-all-cut-paste behavior in Chrome. It's still broken in Firefox
because of selection-related issues, but I'll split that out into a
different bug report.