utils/git: quieter git --version.
utils/git: quieter git --version.

Let's avoid printing this unless `--debug` (to avoid
`brew bundle check -v` outputting this).

Fixes https://github.com/Homebrew/homebrew-bundle/issues/865

claui/brew,Homebrew/brew,Homebrew/brew,vitorgalvao/brew,nandub/brew,EricFromCanada/brew,vitorgalvao/brew,sjackman/homebrew,nandub/brew,JCount/brew,Homebrew/brew,Homebrew/brew,sjackman/homebrew,nandub/brew,JCount/brew,sjackman/homebrew,nandub/brew,MikeMcQuaid/brew,JCount/brew,claui/brew,EricFromCanada/brew,EricFromCanada/brew,sjackman/homebrew,vitorgalvao/brew,vitorgalvao/brew,claui/brew,MikeMcQuaid/brew,JCount/brew,EricFromCanada/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,claui/brew