Fix the exclude_tree argument in get_terms(), which fixes the exclude argument in wp_list_categories().

This was a 3.7 regression caused by [25162].

props dd32.
see #25710 for trunk.

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git-svn-id: http://core.svn.wordpress.org/trunk@25892 1a063a9b-81f0-0310-95a4-ce76da25c4cd