cmd/update: follow GitHub API redirects.
cmd/update: follow GitHub API redirects.

Fixes #4867

Linuxbrew/brew,EricFromCanada/brew,EricFromCanada/brew,Linuxbrew/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,nandub/brew,vitorgalvao/brew,konqui/brew,vitorgalvao/brew,claui/brew,EricFromCanada/brew,konqui/brew,konqui/brew,sjackman/homebrew,vitorgalvao/brew,Linuxbrew/brew,sjackman/homebrew,sjackman/homebrew,MikeMcQuaid/brew,reitermarkus/brew,JCount/brew,nandub/brew,claui/brew,DomT4/brew,reitermarkus/brew,DomT4/brew,claui/brew,DomT4/brew,Homebrew/brew,maxim-belkin/brew,nandub/brew,JCount/brew,claui/brew,mahori/brew,DomT4/brew,Homebrew/brew,Linuxbrew/brew,reitermarkus/brew,EricFromCanada/brew,reitermarkus/brew,mahori/brew,JCount/brew,nandub/brew,maxim-belkin/brew,maxim-belkin/brew,konqui/brew,Homebrew/brew,sjackman/homebrew,MikeMcQuaid/brew,mahori/brew,Homebrew/brew,JCount/brew,vitorgalvao/brew,mahori/brew