urldecode() the incoming $file in the theme editor. see [20313] when the...

urldecode() the incoming $file in the theme editor. see [20313] when the encode was added. see #2994 for the original bug report. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20314 1a063a9b-81f0-0310-95a4-ce76da25c4cd