bug#19572: 24.4; M-x comint-history-isearch-backward doesn't search multi-line history entries

[João Távora]

   emacs -Q
   M-x ielm RET
   'foo RET
   'bar RET
   '(baz M-j bin) RET
   M-x comint-history-isearch-backward RET b a z

expected the multi-line entry to be restored to input, but
comint.el fails to find it.

This is because `comint-history-isearch-search' uses

    (goto-char (comint-line-beginning-position))

I think it should use `comint-goto-process-mark'. comint.el is a big
beast but I think this makes sense. Should I fix it thusly? (unless
anyone beats me to it)

João