Pass a null device instead of explicitly closing stderr

When calling git any error messages should be suppressed (e.g. if an directory
is not inside a git repository).

Closing the stderr output when calling `IO.popen` works fine on POSIX systems,
but on Windows it does not and spits out a warning for every invocation:

```
git.rb:44: warning: cannot close fd before spawn
```

Fixes #517.

lib/colorls/git.rb

@@ -52,7 +52,7 @@ module ColorLS
 
       def git_prefix(repo_path)
         [
-          IO.popen(['git', '-C', repo_path, 'rev-parse', '--show-prefix'], err: :close, &:gets)&.chomp,
+          IO.popen(['git', '-C', repo_path, 'rev-parse', '--show-prefix'], err: File::NULL, &:gets)&.chomp,
           $CHILD_STATUS.success?
         ]
       rescue Errno::ENOENT

spec/color_ls/git_spec.rb

@@ -39,4 +39,10 @@ RSpec.describe ColorLS::Git do
       expect(subject.status('/repo/')).to include('subdir' => Set['M', 'D'])
     end
   end
+
+  context 'determining the git status' do
+    it 'does not output to stderr' do
+      expect { subject.status('.') }.not_to output.to_stderr
+    end
+  end
 end