scm/git: more old OS X versions tweaks.
scm/git: more old OS X versions tweaks.

- unset IFS to fix version parsing on older Bashes
- reuse existing HOMEBREW_MACOS_VERSION if it’s available
- set rather than unset popup_stub

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