download_strategy: ensure fixed commit hash length
download_strategy: ensure fixed commit hash length

The length of the commit hash returned for `--short` can vary depending
on user configuration. Make sure this works independently of what might
have been configured via a user's `.gitconfig`. This also fixes the
failing `GitDownloadStrategyTests#test_last_commit` test for such users.

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