github_packages: change tap_git_head to tap_git_revision, add fallback.
github_packages: change tap_git_head to tap_git_revision, add fallback.

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