bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Gregory Heytings]

> where I can even reduce the regexp down to "[-._[:alnum:]]*\t*=".
>
> Looks like we're missing a case in our backtracking-elimination code.

Apparently we're doing the same thing at the same moment ;-)

I simplified it down to:

(with-current-buffer (get-buffer-create "*bug*")
  (erase-buffer)
  (insert (make-string 266666 ?n))
  (goto-char (point-min))
  (looking-at "[[:alpha:]]*=*"))

This fails with 266666, and succeeds with 266665.