org-datetree.el: Code cleanup
* lisp/org-datetree.el (org-datetree--find-create): New function.
(org-datetree-find-year-create, org-datetree-find-month-create,
org-datetree-find-day-create): Removed functions
(org-datetree-find-date-create): Use `org-datetree--find-create' instead
of removed functions. Use calendar extract functions.
(org-datetree-insert-line): Do more formatting in `format-time-string'
since we call it anyway
* testing/lisp/test-org-datetree.el (test-org-datetree/find-date-create):
Test if new entries are put at the right place.