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.
