title: How to solve 'error: add_cacheinfo failed to refresh for path' when merging at a project containing a submodule
body: <p>I have a project with a submodule. When trying to merge a branch (named <code>release</code>) into <code>master</code>, git identified some conflicts and raised the following error:</p>
  <pre><code>Fast-forwarding submodule path/to/submodule
  Auto-merging path/to/submodule
  error: add_cacheinfo failed to refresh for path 'path/to/submodule'; merge aborting.
  </code></pre>
  <p>It completely stopped the merge, as no evidence of it is shown with <code>git status</code>, although I can see some files from the branch <code>release</code>. If I run <code>git merge --abort</code> it also complains with:</p>
  <pre><code>fatal: There is no merge to abort (MERGE_HEAD missing).
  </code></pre>