Set GIT_DIR when staging git clones

Some build scripts want to inspect the git history, e.g. to determine
version strings. Rather than copy the repository into the build tree,
just setting GIT_DIR is enough for things to "just work".

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/download_strategy.rb

@@ -383,6 +383,7 @@ class GitDownloadStrategy < AbstractDownloadStrategy
         safe_system 'git', 'submodule', '--quiet', 'foreach', '--recursive', sub_cmd
       end
     end
+    ENV['GIT_DIR'] = cached_location+'.git'
   end
 end