scm/git: hide prefix cd failure warning.
scm/git: hide prefix cd failure warning.

This may happen with certain checkout configurations (such as on our CI
boxes).

Also, rename the variables to be clearer about what they reference.

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