[BUG] Use of period in org-read-date broken

[Matt Lundin <mdl@imapmail.org>]

The commit e802936f4b39f8b1e56cc015f34aa655565afe7f broke the use of the
period at the beginning of the minibuffer as a means to jump to the
current day.

Steps to replicate:

1. Evaluate (org-read-date)

2. Use Shift-Left to go to the previous date.

3. Type a period at beginning of minibuffer prompt.

Expected behavior:

The calendar should jump to the current day and no period should be
inserted in the minibuffer. That is the behavior that always worked in
the past and the behavior that is intended in lines 16772 to 16777 of
org.el.

Actual behavior:

A period is inserted in the minibuffer and the calendar does not jump to
the current day.

Here is the problematic commit:

--8<---------------cut here---------------start------------->8---
commit e802936f4b39f8b1e56cc015f34aa655565afe7f
Author: Kyle Meyer <kyle@kyleam.com>
Date:   Sun Sep 27 12:27:52 2015 -0400

    Pass LIMIT argument to org-looking-back
    
    * lisp/org-mouse.el (org-mouse-insert-item):
    (org-mouse-context-menu):
    * lisp/org-pcomplete.el (org-thing-at-point):
    * lisp/org.el (org-read-date-minibuffer-local-map):
    (org-timestamp-change): Provide LIMIT argument to org-looking-back.
    * lisp/org.el (org-read-date-minibuffer-local-map):
    (org-insert-heading): Replace looking-back with org-looking-back.
    
    As of Emacs 25, the LIMIT argument to looking-back is advertised as a
    required argument.
--8<---------------cut here---------------end--------------->8---

Thanks,
Matt