In some cases the completion we come up with may be unexpected, e.g.
if you have files like
/etc/realfile
and
/etc/wrongfile
and enter "/etc/gile", it will accept "wrongfile" because "g" and
"ile" are in there - it's a substring insertion match.
The underlying cause was a typo, so it should be easy to go back.
So we do a bit of magic and let "cancel" undo, but only right after a
completion was accepted via complete or complete-and-search.
That means that just reflexively pressing escape would, by default, get you back to
the old token and let you fix your mistake.
We don't do this when the completion was accepted via the pager,
because 1. there's more of a chance to see the problem there and 2.
it's harder to redo in that case.
Fixes#7433.
