update.sh: always use stable branch for tags.
update.sh: always use stable branch for tags.

This avoids creating a new branch that’ll never be deleted for each tag
and differentiates between the `master` and `stable` branches.

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