breadth-first search for dependents upgrade
breadth-first search for dependents upgrade

Fixes Homebrew/brew#6386

claui/brew,vitorgalvao/brew,vitorgalvao/brew,sjackman/homebrew,claui/brew,Homebrew/brew,mahori/brew,vitorgalvao/brew,vitorgalvao/brew,MikeMcQuaid/brew,Homebrew/brew,sjackman/homebrew,claui/brew,konqui/brew,reitermarkus/brew,reitermarkus/brew,EricFromCanada/brew,mahori/brew,MikeMcQuaid/brew,konqui/brew,nandub/brew,EricFromCanada/brew,Homebrew/brew,mahori/brew,Homebrew/brew,EricFromCanada/brew,MikeMcQuaid/brew,JCount/brew,JCount/brew,JCount/brew,konqui/brew,nandub/brew,JCount/brew,nandub/brew,nandub/brew,MikeMcQuaid/brew,reitermarkus/brew,claui/brew,EricFromCanada/brew,konqui/brew,sjackman/homebrew,mahori/brew,reitermarkus/brew,sjackman/homebrew