print-cmd -x includes instead of excluding

Example command line:

piglit print-cmd --format="{name}" quick -x arb_shader_image_load_store

Expected results:

[ list of tests in quick.py but nothing matching arb_shader_image_load_store ]

Actual results:

spec@arb_shader_image_load_store@atomicity
spec@arb_shader_image_load_store@bitcast
spec@arb_shader_image_load_store@coherency
spec@arb_shader_image_load_store@dead-fragments
spec@arb_shader_image_load_store@early-z
spec@arb_shader_image_load_store@host-mem-barrier
spec@arb_shader_image_load_store@indexing
spec@arb_shader_image_load_store@invalid
spec@arb_shader_image_load_store@layer
spec@arb_shader_image_load_store@level
spec@arb_shader_image_load_store@max-images
spec@arb_shader_image_load_store@max-size
spec@arb_shader_image_load_store@minmax
spec@arb_shader_image_load_store@qualifiers
spec@arb_shader_image_load_store@restrict
spec@arb_shader_image_load_store@semantics
... etc, only arb_shader_image_load_store matches listed