Stop mungegithub from waiting for mergeability if PR is already merged.
Stop mungegithub from waiting for mergeability if PR is already merged.

Mungegithub waits for mergeability to be non-nil before returning
mergeability, but mergeability is nil after PR is merged so mungegithub
waits for the full (~62 second) time out before failing. This commit
fixes this by reporting a failed mergeability lookup if the PR is
merged already, causing the submit-queue to skip it.

kubernetes/test-infra,jlowdermilk/test-infra,fejta/test-infra,spxtr/test-infra,kubernetes/test-infra,BenTheElder/test-infra,brahmaroutu/test-infra,pwittrock/test-infra,krzyzacy/test-infra,spxtr/test-infra,kewu1992/test-infra,krousey/test-infra,cblecker/test-infra,grodrigues3/test-infra,ixdy/kubernetes-test-infra,abgworrall/test-infra,grodrigues3/test-infra,jessfraz/test-infra,kubernetes/test-infra,michelle192837/test-infra,krousey/test-infra,piosz/test-infra,kewu1992/test-infra,jlowdermilk/test-infra,cjwagner/test-infra,lavalamp/test-infra,dims/test-infra,michelle192837/test-infra,pwittrock/test-infra,cjwagner/test-infra,cblecker/test-infra,pwittrock/test-infra,cblecker/test-infra,shashidharatd/test-infra,krzyzacy/test-infra,mindprince/test-infra,cblecker/test-infra,BenTheElder/test-infra,jlowdermilk/test-infra,cjwagner/test-infra,monopole/test-infra,kubernetes/test-infra,spxtr/test-infra,grodrigues3/test-infra,kargakis/test-infra,michelle192837/test-infra,monopole/test-infra,cblecker/test-infra,abgworrall/test-infra,BenTheElder/test-infra,rmmh/kubernetes-test-infra,jessfraz/test-infra,BenTheElder/test-infra,mindprince/test-infra,abgworrall/test-infra,brahmaroutu/test-infra,pwittrock/test-infra,brahmaroutu/test-infra,dims/test-infra,grodrigues3/test-infra,dims/test-infra,jessfraz/test-infra,foxish/test-infra,fejta/test-infra,grodrigues3/test-infra,kargakis/test-infra,rmmh/kubernetes-test-infra,krousey/test-infra,foxish/test-infra,nlandolfi/test-infra-1,jlowdermilk/test-infra,shashidharatd/test-infra,mindprince/test-infra,fejta/test-infra,kargakis/test-infra,nlandolfi/test-infra-1,jessfraz/test-infra,nlandolfi/test-infra-1,abgworrall/test-infra,shyamjvs/test-infra,mindprince/test-infra,monopole/test-infra,fejta/test-infra,michelle192837/test-infra,mindprince/test-infra,cjwagner/test-infra,lavalamp/test-infra,lavalamp/test-infra,krousey/test-infra,piosz/test-infra,dims/test-infra,shyamjvs/test-infra,nlandolfi/test-infra-1,lavalamp/test-infra,shyamjvs/test-infra,fejta/test-infra,krzyzacy/test-infra,foxish/test-infra,krousey/test-infra,shyamjvs/test-infra,foxish/test-infra,ixdy/kubernetes-test-infra,rmmh/kubernetes-test-infra,jlowdermilk/test-infra,shashidharatd/test-infra,kubernetes/test-infra,pwittrock/test-infra,ixdy/kubernetes-test-infra,piosz/test-infra,kubernetes/test-infra,rmmh/kubernetes-test-infra,dims/test-infra,ixdy/kubernetes-test-infra,brahmaroutu/test-infra,cjwagner/test-infra,krzyzacy/test-infra,michelle192837/test-infra,shyamjvs/test-infra,monopole/test-infra,ixdy/kubernetes-test-infra,jlowdermilk/test-infra,spxtr/test-infra,piosz/test-infra,rmmh/kubernetes-test-infra,kargakis/test-infra,michelle192837/test-infra,cblecker/test-infra,lavalamp/test-infra,BenTheElder/test-infra,nlandolfi/test-infra-1,abgworrall/test-infra,fejta/test-infra,kewu1992/test-infra,kargakis/test-infra,BenTheElder/test-infra,kargakis/test-infra,krzyzacy/test-infra,brahmaroutu/test-infra,shyamjvs/test-infra,shashidharatd/test-infra,jessfraz/test-infra,monopole/test-infra,lavalamp/test-infra,dims/test-infra,piosz/test-infra,spxtr/test-infra,jessfraz/test-infra,foxish/test-infra,cjwagner/test-infra,kewu1992/test-infra,krzyzacy/test-infra,shashidharatd/test-infra,brahmaroutu/test-infra,kewu1992/test-infra,monopole/test-infra