Avoid stomping of bulk postdata inside the bulk_edit_posts() loop.

Merges [27990] to the 3.8 branch.

props kovshenin.
see [27964], see #27452.

Built from https://develop.svn.wordpress.org/branches/3.8@27991


git-svn-id: http://core.svn.wordpress.org/branches/3.8@27821 1a063a9b-81f0-0310-95a4-ce76da25c4cd