get_post() hits PAGE cache before POST cache

[markjaquith]

Every call to something like current_user_can('edit_post', $ID) (once for each post in default theme) calls get_post(), which checks the PAGE cache before checking the POST cache.

This causes unnecessary cache misses. We should assume that they're using the function correctly and try hitting the POST cache first.

[markjaquith]

(In [4847]) get_post() should check post cache before page cache. fixes #3724