avoid calling into CPython when updating a set with an empty set
avoid calling into CPython when updating a set with an empty set

fperez/cython,mcanthony/cython,hickford/cython,cython/cython,c-blake/cython,c-blake/cython,encukou/cython,encukou/cython,da-woods/cython,acrispin/cython,c-blake/cython,andreasvc/cython,dahebolangkuan/cython,hickford/cython,fabianrost84/cython,dahebolangkuan/cython,slonik-az/cython,fabianrost84/cython,madjar/cython,fperez/cython,slonik-az/cython,da-woods/cython,roxyboy/cython,mrGeen/cython,da-woods/cython,fabianrost84/cython,marscher/cython,mrGeen/cython,cython/cython,JelleZijlstra/cython,hickford/cython,fperez/cython,roxyboy/cython,achernet/cython,slonik-az/cython,achernet/cython,mcanthony/cython,ChristopherHogan/cython,slonik-az/cython,madjar/cython,marscher/cython,hickford/cython,fabianrost84/cython,madjar/cython,mcanthony/cython,roxyboy/cython,acrispin/cython,marscher/cython,andreasvc/cython,dahebolangkuan/cython,hhsprings/cython,c-blake/cython,hhsprings/cython,encukou/cython,andreasvc/cython,mcanthony/cython,achernet/cython,fperez/cython,mrGeen/cython,hhsprings/cython,roxyboy/cython,c-blake/cython,encukou/cython,acrispin/cython,JelleZijlstra/cython,dahebolangkuan/cython,cython/cython,roxyboy/cython,achernet/cython,ChristopherHogan/cython,encukou/cython,hhsprings/cython,mrGeen/cython,dahebolangkuan/cython,achernet/cython,andreasvc/cython,fperez/cython,marscher/cython,mcanthony/cython,mrGeen/cython,ChristopherHogan/cython,JelleZijlstra/cython,madjar/cython,scoder/cython,slonik-az/cython,scoder/cython,madjar/cython,acrispin/cython,fabianrost84/cython,JelleZijlstra/cython,scoder/cython,hhsprings/cython,scoder/cython,da-woods/cython,cython/cython,JelleZijlstra/cython,andreasvc/cython,acrispin/cython,marscher/cython,hickford/cython