Ignore ~/.gitconfig

I use `commit.gpgsign=true`, so I want to ignore it in tests.

Author: znz

tool/test/test_commit_email.rb

@@ -13,7 +13,11 @@ def setup
       git('init', '--initial-branch=master')
       git('config', 'user.name', 'Jóhän Grübél')
       git('config', 'user.email', 'johan@example.com')
-      env = { 'GIT_AUTHOR_DATE' => '2025-10-08T12:00:00Z', 'TZ' => 'UTC' }
+      env = {
+        'GIT_AUTHOR_DATE' => '2025-10-08T12:00:00Z',
+        'GIT_CONFIG_GLOBAL' => IO::NULL,
+        'TZ' => 'UTC',
+      }
       git('commit', '--allow-empty', '-m', 'New repository initialized by cvs2svn.', env:)
       git('commit', '--allow-empty', '-m', 'Initial revision', env:)
       git('commit', '--allow-empty', '-m', 'version　1.0.0', env:)