update.sh: don't restore branch to stable.
update.sh: don't restore branch to stable.

Only run for Homebrew developers so assume they don’t want to switch
back to an old stable branch.

Fixes #1141.

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