This should confirm that issue #835 is fixed where a submodule that
is only declared in the .gitmodules file was not accessible via the
submodule APIs.
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 fixes up a number of problems flagged by valgrind and also
cleans up the internal `git_submodule` allocation handling
overall with a simpler model.
