GitPython incorrectly populates blobs for submodule changes, when using diff

Our weblate_update_git.py script implements an own merge strategy and so it needs to access the elements within the diff, to decide how to merge. Currently we need a workaround for submodule changes, done by remote (main git).

The upstream issue is tracked at: https://github.com/gitpython-developers/GitPython/issues/891

If that is solved, we can remove the workaround.

For puppet-tails@a36884e96ce36d3c5e1c348d9813fffce46e4350

<code class="python">
480             if not isWikiPo(path):
481                 try:
482                     blob = i.b_blob
483                     index.updateFile(path, blob)
484                 except ValueError:
                        [...]
</code>

Original created by @Hefee on 16862 (Redmine)