bug#22295: viper-mode undo bug introduced between Nov 10 and Nov 14

[Jim Meyering]

Hello,
I noticed that viper-mode's "undo" ('u') command began to undo too much
and was able quickly to determine that it worked fine with my snapshot
built from git master some time on Nov 10, yet that it began to undo
too much four days later.

To demonstrate the problem (without risking changing anything in your
home directory), run this:

  mkdir /tmp/x && HOME=/tmp/x emacs -Q -f viper-mode -nw
~/previously-nonexistent-file

then respond "y", "y", "5" to get past the "viperize" setup questions.
To reproduce the error, insert two lines, terminating each "insertion" with ESC,
so that each is recorded as a separate undo'able operation. I.e., type this

   a 1 ESC

to create the first line, then

  o 2 ESC

to create the second.
Finally, hit "u" to undo creation of the second and you'll see that it undoes
both operations, erasing both lines.  This is rather disruptive when that first
bit of text was a long paragraph or two -- the novice may think that it's lost,
because redo does not restore it -- however, it is available in emacs's
yank buffer.

Thanks for tending emacs,

Jim