Added in WP 3.7 [25123], the 'taxonomy_parent_dropdown_args' filter affected
only the term creation interface. This changeset introduces parity by ensuring
that it is applied when editing terms as well.
The new `$context` parameter indicates whether the filter is being applied in
a 'new' or 'edit' context.
Props TimothyBlynJacobs, DrewAPicture.
Fixes#29853.
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