Ensure example git repos are valid (#1821)

* Ensure example git repos are valid

It was reported in #1817 that the `lotr_migration.git` repository we use
in `test/test_migrate.rb` is not a valid git repository on when clone,
causing the test suite to fail when run locally.

The reason is because there is no `.git/refs` directory, meaning it's
not really a valid git repository at all. I noticed that the `empty.git`
example repository has the same problem.

This commit simply ensures that the directory structure of these example
repositories are persisted in git.

* Run `test_migrate` tests on CI

Now that we've resolved the issue with the invalid git repository in the
parent commit, we can run all of the tests in our CI environment.

test/test_migrate.rb

@@ -25,8 +25,7 @@ def load_script(**args)
   end
 end
 
-unless ENV['CI']
+context '4.x -> 5.x tag migrator' do
-  context '4.x -> 5.x tag migrator' do
   include Rack::Test::Methods
 
   setup do
@@ -82,5 +81,4 @@ unless ENV['CI']
   teardown do
     FileUtils.rm_rf(@path)
   end
-  end
 end