Allow cloning subtrees while shifting them backward in time

* lisp/org.el (org-clone-subtree-with-time-shift): Accept a negative
value to shift the timestamp backward in time.
* testing/lisp/test-org.el (test-org/clone-with-time-shift): Add test.

Reported-by: Scott Randby <srandby@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2019-01/msg00151.html>

Signed-off-by: Yasushi SHOJI <yasushi.shoji@gmail.com>

etc/ORG-NEWS

@@ -104,6 +104,11 @@ A 'native option was added to org-highlight-latex-and-related. It
 matches the same structures than 'latex but it calls
 org-src-font-lock-fontify-block instead, thus bringing about full
 LaTeX font locking.
+*** ~org-clone-subtree-with-time-shift~ learnt to shift backward in time
+=<C-c C-x c>= (~org-clone-subtree-with-time-shift~) now takes a
+negative value as a valid repeater to shift time stamps in backward
+in cloned subtrees.  You can give, for example, ‘-3d’ to shift three
+days in the past.
 
 ** New functions
 *** ~org-dynamic-block-insert-dblock~

lisp/org.el

@@ -8452,7 +8452,7 @@ with the original repeater."
 		"")))			;No time shift
 	 (doshift
 	  (and (org-string-nw-p shift)
-	       (or (string-match "\\`[ \t]*\\+?\\([0-9]+\\)\\([dwmy]\\)[ \t]*\\'"
+	       (or (string-match "\\`[ \t]*\\([\\+\\-]?[0-9]+\\)\\([dwmy]\\)[ \t]*\\'"
 				 shift)
 		   (user-error "Invalid shift specification %s" shift)))))
     (goto-char end-of-tree)

testing/lisp/test-org.el

@@ -1496,6 +1496,18 @@
 	    (replace-regexp-in-string
 	     "\\( [.A-Za-z]+\\)\\( \\+[0-9][hdmwy]\\)?>" "" (buffer-string)
 	     nil nil 1))))
+  ;; Clone repeating once in backward.
+  (should
+   (equal "\
+* H1\n<2015-06-21>
+* H1\n<2015-06-19>
+* H1\n<2015-06-17 +1w>
+"
+	  (org-test-with-temp-text "* H1\n<2015-06-21 Sun +1w>"
+	    (org-clone-subtree-with-time-shift 1 "-2d")
+	    (replace-regexp-in-string
+	     "\\( [.A-Za-z]+\\)\\( \\+[0-9][hdmwy]\\)?>" "" (buffer-string)
+	     nil nil 1))))
   ;; Clone non-repeating zero times.
   (should
    (equal "\