utils/git: various improvement
utils/git: various improvement

* Use `ENV/scm/git` to check whether git is installed.
* Use `brew install git` to enforce pre-install checks such as sudo
  check.

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