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.

This reworks the diff API to separate the steps of producing
a diff descriptions from formatting the diff.  This will allow
us to share diff output code with the various diff creation
scenarios and will allow us to implement rename detection as
an optional pass that can be run on a diff list.

Notes API

This commit adds basic git notes support to libgit2, namely:

* git_note_read
* git_note_message
* git_note_oid
* git_note_create
* git_note_remove

In the long run, we probably want to provide some convenience callback
mechanism for merging and moving (filter-branch) notes.

Signed-off-by: schu <schu-github@schulog.org>

This is legacy compat stuff for when `deflateBound` is not defined, but
we're not embedding zlib and that function is always available. Kill
that with fire.

Signed-off-by: schu <schu-github@schulog.org>

- Hide the remaining transports code
- Drop `git_headarray`, switch to using a callback to list refs. Makes
the code cleaner.

See `global.c` for a description of what we're doing.

When libgit2 is built with GIT_THREADS support, the threading system
must be explicitly initialized with `git_threads_init()`.

Signed-off-by: Sven Strickroth <email@cs-ware.de>