In `get_default_post_to_edit()`, let 'post_name' be passed to filter.

Previously, it was being overridden after the default post had been inserted
into the database, making it cumbersome to override with the 'wp_insert_post_data'
filter.

Props danielbachhuber.
Fixes #29847.

git-svn-id: https://develop.svn.wordpress.org/trunk@30626 602fd350-edb4-49c9-b593-d223f7449a82