Fix timestamp-based sorting of tags-based entries in agenda
* lisp/org.el (org-scan-tags): Fix agenda org tags scans to properly
add timestamp property, completely analogously to
org-agenda-get-todos.
* lisp/org-agenda.el (org-agenda-entry-get-agenda-timestamp,
org-agenda-get-todos): Factored timestamp retrieval code out to
separate function org-agenda-entry-get-agenda-timestamp from
org-agenda-get-todos.
Before this fix, timestamps were ignored when sorting agenda views of
the 'tags' and 'tags-todo' types.
TINYCHANGE