Don't store a revision for the initial call to wp_insert_post(), only on saves.

In core UI (and XML-RPC), we first create an auto-draft via get_default_post_to_edit(), which means the first "save" is in practice the first revision.

This reverts [23842].

fixes #24708.



git-svn-id: https://core.svn.wordpress.org/trunk@24650 1a063a9b-81f0-0310-95a4-ce76da25c4cd