add undo to query-replace choices

[Dan Jacobson]

Sure there are lots of things one can do when query replacing,

  Type Space or `y' to replace one match, Delete or `n' to skip to next,
  RET or `q' to exit, Period to replace one match and exit...

But say what if fumblefingers typed his y's too fast and regrets the
one he has just typed in and is starting at.

SPC C-/ C-x <escape> <escape> is enough to get him fixed up, but maybe
you want to add a "u" command to undo the last (in/)action.
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