cmd: construct context properly
cmd: construct context properly

This is a bug that cmd/gb/main.go and cmd/cmd.go duplicate this logic.

hyper-carrot/gb,constabulary/gb,tbruyelle/gb,davidkaya/gb,mopemope/gb,mopemope/gb,Luit/gb,inconshreveable/gb,URXtech/gb,davidkaya/gb,lparkes/gb,tianon/gb,constabulary/gb,inconshreveable/gb,hyper-carrot/gb,guiquanz/gb,tianon/gb,ches/gb,mopemope/gb,constabulary/gb,ches/gb,tbruyelle/gb