Allow inserting diary entries last in date tree
* org-agenda.el (org-agenda-insert-diary-strategy): Add new value
'date-tree-last.
(org-agenda-insert-diary-make-new-entry): Handle
`org-agenda-insert-diary-strategy' set to 'date-tree-last.
To allow for diary entries to be entered in time order in the date tree,
add a new value to `org-agenda-insert-diary-strategy' that allows for
this. The code for handling this value, 'date-tree-last, is a bit
tricky, as we need to keep track of whether the date-tree already had
one or more entries for the given date.