bug#17236: undo-only hangs when nils double up

[Barry OReilly]

When there are consecutive nils in buffer-undo-list, undo-only can
hang. For example, if buffer-undo-list begins with (nil nil ...), then
the first undo-only succeeds, changes nothing, and inserts an identity
mapping into undo-equiv-table. The second undo-only hangs because the
identity mapping causes an infinite recursive lookup in
undo-equiv-table.

Perhaps the bug is in the Emacs package (Evil in this case) that
doubles up the nils. I ask here because the undo code already seems to
try to account for these to some extent. eg there are a few of these
in the undo function:

  (while (eq (car list) nil)
         (setq list (cdr list)))

Is this an Emacs bug or an Evil bug?