utils/git: cache less aggressively.
utils/git: cache less aggressively.

This should avoid weird test ordering failures with specs on Linux (but
there's no real reason this can't occur on macOS too).

aw1621107/brew,vitorgalvao/brew,Homebrew/brew,pseudocody/brew,reitermarkus/brew,mgrimes/brew,ilovezfs/brew,JCount/brew,claui/brew,Linuxbrew/brew,JCount/brew,mgrimes/brew,Linuxbrew/brew,DomT4/brew,konqui/brew,claui/brew,Homebrew/brew,pseudocody/brew,konqui/brew,sjackman/homebrew,mahori/brew,Homebrew/brew,DomT4/brew,ilovezfs/brew,sjackman/homebrew,ilovezfs/brew,reitermarkus/brew,Linuxbrew/brew,Homebrew/brew,hanxue/linuxbrew,maxim-belkin/brew,konqui/brew,vitorgalvao/brew,mahori/brew,MikeMcQuaid/brew,mgrimes/brew,aw1621107/brew,sjackman/homebrew,jmsundar/brew,alyssais/brew,DomT4/brew,reitermarkus/brew,maxim-belkin/brew,MikeMcQuaid/brew,reitermarkus/brew,EricFromCanada/brew,claui/brew,reelsense/brew,nandub/brew,nandub/brew,reelsense/brew,Linuxbrew/brew,DomT4/brew,nandub/brew,palxex/brew,EricFromCanada/brew,pseudocody/brew,palxex/brew,jmsundar/brew,EricFromCanada/brew,aw1621107/brew,hanxue/linuxbrew,vitorgalvao/brew,alyssais/brew,hanxue/linuxbrew,MikeMcQuaid/brew,alyssais/brew,nandub/brew,palxex/brew,EricFromCanada/brew,vitorgalvao/brew,sjackman/homebrew,maxim-belkin/brew,JCount/brew,mahori/brew,claui/brew,MikeMcQuaid/brew,mahori/brew,reelsense/brew,konqui/brew,JCount/brew,jmsundar/brew