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.

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git-svn-id: http://core.svn.wordpress.org/branches/3.7@27713 1a063a9b-81f0-0310-95a4-ce76da25c4cd