update: make git rebase quiet on Mojave system Git.
update: make git rebase quiet on Mojave system Git.

Fix regression from https://github.com/Homebrew/brew/pull/7260.

This code makes me sad.

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