Rename submodule into subproject in diff to match CGit behavior

When generating a diff in a repository with dirty submodules, CGit is using
the subproject terminology. I couldn't find the reasoning why or since when it
has been doing it but https://stackoverflow.com/q/4873980/2996250 seems to
indicate it has been the case since 2011 at least.

It has no real impacts as submodules information in diff are mostly ignored by
git apply but this makes Dulwich behavior closer to CGit one.