After buffer modification - in particular after deletion - adjacent
selection history entries may correspond to the same effective
selection when applied to the current buffer. This means that we
sometimes need to press <c-h> multiple times to make one visible
change. This is not what the user expects, so let's keep walking the
selection history until we hit an actual change.
Alternatively, we could minimize the selection history after buffer
changes but I think that would make the it worse after content
undo+redo.
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.
