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.