Fix highlighting in sparse-tree

* org.el (org-scan-tags): Fix highlighting in sparse-tree.

`org-get-heading' uses `org-heading-regexp' introduced by
dfcb6faef11a2439b56b18a6289803361d402130 but `org-scan-tags' assumed
match groups of the old regular expression.

This should fix a bug reported by Suvayu Ali in
<20120209032255.431e8197@kuru.dyndns-at-home.com>.

lisp/org.el

@@ -12879,7 +12879,7 @@ headlines matching this string."
 	      (and org-highlight-sparse-tree-matches
 		   (org-get-heading) (match-end 0)
 		   (org-highlight-new-match
-		    (match-beginning 0) (match-beginning 1)))
+		    (match-beginning 1) (match-end 1)))
 	      (org-show-context 'tags-tree))
 	     ((eq action 'agenda)
 	      (setq txt (org-agenda-format-item