upgrade: keg_only before non-keg_only formulae
upgrade: keg_only before non-keg_only formulae

Avoids unnecessary conflicts when a previously non-keg_only formula is
in the way by prioritizing keg_only before non-keg_only formulae.

This change is motivated by the upgrade of gnupg 2.0 to 2.1, since the
latter no longer depends on the gpg-agent formula, which, even if made
keg_only, still causes the link step to fail for 2.1, as gpg-agent's
non-keg version won't have been upgraded to the keg_only version at that
point (alphabetically gnupg precedes gpg-agent).

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