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.
