When checking filesystem permissions for core updates, only check the permissions of the files which we need to alter.

Merges [26014] from 3.8 to the 3.7 branch.

fixes #25771.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@27882 602fd350-edb4-49c9-b593-d223f7449a82