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.

When processing status for a newly checked out repo, it is
possible that there will be submodules that have not yet been
initialized.  The only way to distinguish these from untracked
directories is to have some knowledge of submodules.  This
commit adds a new submodule API which, given a name or path,
can determine if it appears to be a submodule and can give
information about the submodule.