brew.rb: handle -—help for internal/external cmds.
brew.rb: handle -—help for internal/external cmds.

Only display —-help for internal commands and not for external ones;
they can handle the flag themselves.

Closes Homebrew/homebrew#26675.
References Homebrew/homebrew#26755.
Closes Homebrew/homebrew#30300.

jmsundar/brew,konqui/brew,JCount/brew,JCount/brew,muellermartin/dist,vitorgalvao/brew,bfontaine/brew,amar-laksh/brew_sudo,MikeMcQuaid/brew,jmsundar/brew,reitermarkus/brew,Homebrew/brew,alyssais/brew,claui/brew,alyssais/brew,tonyg/homebrew,pseudocody/brew,mahori/brew,tonyg/homebrew,mistydemeo/homebrew,MikeMcQuaid/brew,reelsense/brew,aw1621107/brew,bfontaine/brew,reitermarkus/brew,zmwangx/brew,staticfloat/brew,nandub/brew,MikeMcQuaid/brew,reitermarkus/brew,AnastasiaSulyagina/brew,gregory-nisbet/brew,sjackman/homebrew,tonyg/homebrew,rwhogg/brew,rwhogg/brew,maxim-belkin/brew,AnastasiaSulyagina/brew,DomT4/brew,EricFromCanada/brew,konqui/brew,maxim-belkin/brew,vitorgalvao/brew,nandub/brew,JCount/brew,amar-laksh/brew_sudo,aw1621107/brew,muellermartin/dist,jmsundar/brew,alyssais/brew,rwhogg/brew,pseudocody/brew,DomT4/brew,staticfloat/brew,nandub/brew,hanxue/linuxbrew,claui/brew,Linuxbrew/brew,hanxue/linuxbrew,palxex/brew,ilovezfs/brew,mahori/brew,gregory-nisbet/brew,claui/brew,konqui/brew,toonetown/homebrew,AnastasiaSulyagina/brew,ilovezfs/brew,mgrimes/brew,gordonmcshane/homebrew,mgrimes/brew,mgrimes/brew,amar-laksh/brew_sudo,pseudocody/brew,palxex/brew,EricFromCanada/brew,Linuxbrew/brew,sjackman/homebrew,sjackman/homebrew,claui/brew,Linuxbrew/brew,zmwangx/brew,EricFromCanada/brew,aw1621107/brew,reelsense/brew,sjackman/homebrew,DomT4/brew,Homebrew/brew,Homebrew/brew,muellermartin/dist,hanxue/linuxbrew,mahori/brew,reitermarkus/brew,nandub/brew,mistydemeo/homebrew,vitorgalvao/brew,maxim-belkin/brew,palxex/brew,JCount/brew,EricFromCanada/brew,DomT4/brew,Homebrew/brew,vitorgalvao/brew,reelsense/brew,MikeMcQuaid/brew,Linuxbrew/brew,gordonmcshane/homebrew,bfontaine/brew,staticfloat/brew,zmwangx/brew,gordonmcshane/homebrew,ilovezfs/brew,konqui/brew,toonetown/homebrew,gregory-nisbet/brew,mahori/brew,toonetown/homebrew,mistydemeo/homebrew