Bypass git submodule add/update with git config protocol.file.allow=always option.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

test/rubygems/test_gem_source_git.rb

@@ -63,6 +63,11 @@ class TestGemSourceGit < Gem::TestCase
   end
 
   def test_checkout_submodules
+    # We need to allow to checkout submodules with file:// protocol
+    # CVE-2022-39253
+    # https://lore.kernel.org/lkml/xmqq4jw1uku5.fsf@gitster.g/
+    system(@git, *%W"config --global protocol.file.allow always")
+
     source = Gem::Source::Git.new @name, @repository, "master", true
 
     git_gem "b"