help: show help text for tapped external command
help: show help text for tapped external command

Previously, running e.g. `brew help bundle` when homebrew-bundle was not
tapped would tap homebrew-bundle and then run `brew bundle` showing
`Error: No Brewfile found` instead of the expected help text.

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