brew unlink: don't unlink an unlinked formula
brew unlink: don't unlink an unlinked formula

This caused weird issues in the case that multiple formulae provide
the same file.
DomT4/brew,maxim-belkin/brew,nandub/brew,konqui/brew,nandub/brew,gregory-nisbet/brew,sjackman/homebrew,toonetown/homebrew,JCount/brew,bfontaine/brew,bfontaine/brew,Homebrew/brew,zmwangx/brew,pseudocody/brew,alyssais/brew,amar-laksh/brew_sudo,MikeMcQuaid/brew,vitorgalvao/brew,zmwangx/brew,mgrimes/brew,amar-laksh/brew_sudo,EricFromCanada/brew,rwhogg/brew,MikeMcQuaid/brew,tonyg/homebrew,toonetown/homebrew,tonyg/homebrew,reelsense/brew,alyssais/brew,konqui/brew,gordonmcshane/homebrew,claui/brew,ilovezfs/brew,reitermarkus/brew,Linuxbrew/brew,bfontaine/brew,claui/brew,gordonmcshane/homebrew,staticfloat/brew,rwhogg/brew,Linuxbrew/brew,palxex/brew,pseudocody/brew,mistydemeo/homebrew,muellermartin/dist,ilovezfs/brew,MikeMcQuaid/brew,alyssais/brew,aw1621107/brew,mahori/brew,konqui/brew,Linuxbrew/brew,jmsundar/brew,reitermarkus/brew,claui/brew,JCount/brew,Homebrew/brew,Linuxbrew/brew,gordonmcshane/homebrew,nandub/brew,muellermartin/dist,Homebrew/brew,JCount/brew,reelsense/brew,palxex/brew,amar-laksh/brew_sudo,gregory-nisbet/brew,staticfloat/brew,AnastasiaSulyagina/brew,vitorgalvao/brew,toonetown/homebrew,DomT4/brew,reelsense/brew,pseudocody/brew,EricFromCanada/brew,mgrimes/brew,sjackman/homebrew,palxex/brew,mahori/brew,zmwangx/brew,gregory-nisbet/brew,mgrimes/brew,hanxue/linuxbrew,reitermarkus/brew,muellermartin/dist,jmsundar/brew,DomT4/brew,vitorgalvao/brew,hanxue/linuxbrew,maxim-belkin/brew,konqui/brew,vitorgalvao/brew,staticfloat/brew,jmsundar/brew,maxim-belkin/brew,sjackman/homebrew,MikeMcQuaid/brew,mahori/brew,mistydemeo/homebrew,reitermarkus/brew,hanxue/linuxbrew,EricFromCanada/brew,aw1621107/brew,mistydemeo/homebrew,claui/brew,nandub/brew,AnastasiaSulyagina/brew,JCount/brew,DomT4/brew,mahori/brew,Homebrew/brew,sjackman/homebrew,tonyg/homebrew,rwhogg/brew,AnastasiaSulyagina/brew,aw1621107/brew,ilovezfs/brew,EricFromCanada/brew