Tweak selection-undo interaction with WinDisplay hooks

Each selection undo operation is surrounded by pair of
begin_edition()/end_edition() calls.
The original reason for adding these was that in one of my preliminary
versions, a WinDisplay hook could break an undo chain, even if the
hook did not affect selections at all. This has since been fixed.

By surrounding the undo with begin_edition()/end_edition(), try to
ensure that any selection modification that happens in a WinDisplay
hook would not break the undo chain. Essentially this means that,
after using <c-h> to undo a buffer change, this was meant to
make sure that <c-k> could redo that buffer change.

However, it turns out this actually doesn't work.  The attached test
case triggers an assertion.  As described in the first paragraph,
the only real-world motivation for this is gone, so let's simplify
the behavior.
The assertion fix means that we can test the next commit better.

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