title: Git diff ignores textconv in submodules
body: <p>My project is structured like so:</p>
  <pre><code>/project
    - file.json
    - /submodule
      - submodule-file.json
  </code></pre>
  <p>Where the <code>/submodule</code> directory is a git submodule.</p>
  <p>I have set up textconv to do some simple pre-processing of json files for diffing - my local <code>.git/config</code> file contains</p>
  <pre><code>[diff &quot;json&quot;]
      textconv = jq -S .
  </code></pre>
  <p>This works great for files in the main project - <code>git diff file.json</code> works as expected. But it doesn't work for files in the submodule.</p>
  <p><code>git diff submodule/submodule-file.json</code> produces no output at all.</p>
  <p><code>cd submodule &amp;&amp; git diff submodule-file.json</code> produces a diff without the textconv applied.</p>
  <p>I would prefer not to make these settings global, and would be happy if I could have them only apply to the submodule.</p>
  <p>Is this possible?</p>
  <p>git version 2.26.1</p>