Avoid some code duplication in diary-lib.el.

Avoid some code duplication in diary-lib.el.

* lisp/calendar/diary-lib.el (diary-included-files): Doc fix.
(diary-include-files): New function, extracted from
diary-include-other-diary-files and diary-mark-included-diary-files.
(diary-include-other-diary-files, diary-mark-included-diary-files):
Just call diary-include-files.
(diary-mark-entries): Reset diary-included-files on first call.