This adds tests for diffs with submodules in them and (perhaps
unsurprisingly) requires further fixes to be made.  Specifically,
this fixes:

- when considering if a submodule is dirty in the workdir, it was
  being treated as dirty even if only the index was dirty.
- git_diff_patch_to_str (and git_diff_patch_print) were "printing"
  the headers for files (and submodules) that were unmodified or
  had no meaningful content.
- added comment to previous fix and removed unneeded parens.