#19932: Pass post ID to post revision field filter

When viewing a post revision, for each field the filter

_wp_post_revision_field_xx

is called. This is passed the field name and value as parameters but not the post ID that it’s working with, this allows you to modify the value based on it’s currently contents but not based on the post ID i.e. to inject new fields into the diff.

Proposal that the post ID also be passed to this filter rather than having to hack around it by making $left and $right global.