`git_diff_get_patch()` would unconditionally load the patch object and
then simply leak it if the user hadn't requested it. Short-circuit
loading the object if the user doesn't want it.

The rest of the plugs are simply calling the free functions of objects
allocated during the tests.

This removes assertions that prevent us from having an empty
git_config object and then updates some tests that were
dependent on global config state to use an empty config before
running anything.

This fixes both a test that I broke in diff::patch where I was
relying on the current state of the working directory for the
renames test data and fixes an unstable test in diff::rename
where the environment setting for the "diff.renames" config was
being allowed to influence the test results.

Fix #1308

This adds a `git_diff_patch_line_stats()` API that gets the total
number of adds, deletes, and context lines in a patch.  This will
make it a little easier to emulate `git diff --stat` and the like.

Right now, this relies on generating the `git_diff_patch` object,
which is a pretty heavyweight way to get stat information.  At
some future point, it would probably be nice to be able to get
this information without allocating the entire `git_diff_patch`,
but that's a much larger project.

This was just wrong.  Added a test that verifying patch line
numbers even for hunks further into a file and then fixed the
algorithm.  I needed to add a little extra state into the patch
so that I could track old and new file numbers independently,
but it should be okay.

This makes the diff functions that take callbacks both take
the payload parameter after the callback function pointers and
pass the payload as the last argument to the callback function
instead of the first.  This should make them consistent with
other callbacks across the API.

The diff API is not in the parameter order one would expect from
other libgit2 APIs.  This fixes that.

This adds an API to generate a complete single-file patch text
from a git_diff_patch object.

There are a lot of places where the diff API gives the user access
to internal data structures and many of these were being exposed
through non-const pointers.  This replaces them all with const
pointers for any object that the user can access but is still
owned internally to the git_diff_list or git_diff_patch objects.

This will probably break some bindings...  Sorry!

This should restore the ability to include libgit2 headers
in C++ projects.

Cherry picked 2de60205 from
development into new-error-handling.

Update the callback to provide some information related to the file change being processed and the range of the hunk, when applicable.