update: tweak git too old check.
update: tweak git too old check.

Avoid trying to install and warning if system Git is too old but Git
is already installed.

reitermarkus/brew,EricFromCanada/brew,Linuxbrew/brew,maxim-belkin/brew,vitorgalvao/brew,hanxue/linuxbrew,Linuxbrew/brew,nandub/brew,Homebrew/brew,Linuxbrew/brew,sjackman/homebrew,DomT4/brew,hanxue/linuxbrew,JCount/brew,maxim-belkin/brew,MikeMcQuaid/brew,vitorgalvao/brew,reitermarkus/brew,sjackman/homebrew,DomT4/brew,sjackman/homebrew,DomT4/brew,nandub/brew,Homebrew/brew,sjackman/homebrew,konqui/brew,mahori/brew,claui/brew,Linuxbrew/brew,mahori/brew,JCount/brew,maxim-belkin/brew,claui/brew,mgrimes/brew,mgrimes/brew,MikeMcQuaid/brew,alyssais/brew,mahori/brew,JCount/brew,Homebrew/brew,nandub/brew,EricFromCanada/brew,mahori/brew,konqui/brew,claui/brew,alyssais/brew,MikeMcQuaid/brew,ilovezfs/brew,alyssais/brew,hanxue/linuxbrew,DomT4/brew,konqui/brew,vitorgalvao/brew,mgrimes/brew,MikeMcQuaid/brew,vitorgalvao/brew,konqui/brew,JCount/brew,claui/brew,reitermarkus/brew,nandub/brew,ilovezfs/brew,EricFromCanada/brew,Homebrew/brew,reitermarkus/brew,ilovezfs/brew,EricFromCanada/brew