download_strategy: fix git submodule references (#303)
download_strategy: fix git submodule references (#303)

Some Git versions create absolute `gitdir:` pointers in a submodule's
`.git` file, which can be problematic once the resource is staged and
causes various Git operations for those submodules to fail. Work around
this issue by fixing the submodule `.git` files after submodule update.

See Homebrew/homebrew-core#1520 for details and an affected formula.
aw1621107/brew,tonyg/homebrew,muellermartin/dist,vitorgalvao/brew,JCount/brew,Linuxbrew/brew,vitorgalvao/brew,nandub/brew,mahori/brew,claui/brew,gordonmcshane/homebrew,bfontaine/brew,aw1621107/brew,vitorgalvao/brew,amar-laksh/brew_sudo,jmsundar/brew,zmwangx/brew,DomT4/brew,DomT4/brew,konqui/brew,Linuxbrew/brew,gordonmcshane/homebrew,MikeMcQuaid/brew,Homebrew/brew,DomT4/brew,amar-laksh/brew_sudo,mahori/brew,rwhogg/brew,nandub/brew,pseudocody/brew,bfontaine/brew,DomT4/brew,AnastasiaSulyagina/brew,staticfloat/brew,zmwangx/brew,sjackman/homebrew,tonyg/homebrew,reitermarkus/brew,muellermartin/dist,zmwangx/brew,rwhogg/brew,JCount/brew,EricFromCanada/brew,pseudocody/brew,konqui/brew,AnastasiaSulyagina/brew,maxim-belkin/brew,sjackman/homebrew,ilovezfs/brew,JCount/brew,claui/brew,alyssais/brew,mgrimes/brew,claui/brew,MikeMcQuaid/brew,gregory-nisbet/brew,palxex/brew,mahori/brew,hanxue/linuxbrew,EricFromCanada/brew,aw1621107/brew,jmsundar/brew,sjackman/homebrew,reitermarkus/brew,amar-laksh/brew_sudo,Homebrew/brew,EricFromCanada/brew,mgrimes/brew,alyssais/brew,reitermarkus/brew,muellermartin/dist,reelsense/brew,reelsense/brew,palxex/brew,palxex/brew,AnastasiaSulyagina/brew,MikeMcQuaid/brew,hanxue/linuxbrew,alyssais/brew,Homebrew/brew,toonetown/homebrew,mahori/brew,gordonmcshane/homebrew,rwhogg/brew,reelsense/brew,staticfloat/brew,vitorgalvao/brew,hanxue/linuxbrew,ilovezfs/brew,gregory-nisbet/brew,maxim-belkin/brew,gregory-nisbet/brew,EricFromCanada/brew,toonetown/homebrew,MikeMcQuaid/brew,Linuxbrew/brew,pseudocody/brew,JCount/brew,reitermarkus/brew,jmsundar/brew,nandub/brew,tonyg/homebrew,bfontaine/brew,sjackman/homebrew,mgrimes/brew,staticfloat/brew,nandub/brew,toonetown/homebrew,konqui/brew,maxim-belkin/brew,claui/brew,Linuxbrew/brew,ilovezfs/brew,konqui/brew,Homebrew/brew