diagnostic: check homebrew/core git remote.
diagnostic: check homebrew/core git remote.

Check the remote for homebrew/core as well as homebrew/brew as we've
seen a few cases (e.g. #2387) where it was misconfigured.

While we're here, tweak the messaging to tell people what to do and how
to do so with a single command.

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