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.

This adds a new API to the submodule interface that just returns
where information about the submodule was found (e.g. config file
only or in the HEAD, index, or working directory).

Also, the old "refresh" call was potentially keeping some stale
submodule data around, so this simplfies that code and literally
discards the old cache, then reallocates.

The adds a test for the submodule diff capabilities and then
fixes a few bugs with how the output is generated.  It improves
the accuracy of OIDs in the diff delta object and makes the
submodule output more closely mirror the OIDs that will be used
by core git.

When `git_submodule` became an opaque structure, I forgot to add
accessor functions for the fetchRecurseSubmodules config setting.
This fixes that.

This is a big redesign of the git_submodule_status API and the
implementation of the redesigned API.  It also fixes a number of
bugs that I found in other parts of the submodule API while
writing the tests for the status part.

This also fixes a couple of bugs in the iterators that had not
been noticed before - one with iterating when there is a gitlink
(i.e. separate-work-dir) and one where I was treating anything
even vaguely submodule-like as a submodule, more aggressively
than core git does.

This replaces the old submodule API with a new extended API that
supports most of the things that can be done with `git submodule`.

This adds support for a bunch of core.* settings that affect
diff and status, plus fixes up some incorrect implementations
of those settings from before.  Also, this cleans up the
handling of config settings in the new submodules code and
in the old attrs/ignore code.