This is a convenience function to reverse the contents of a vector and a pqueue
in-place.

The pqueue function is useful in the case where we're treating it as a
LIFO queue.

The beginnings of patch application from an existing (diff-created)
git_patch object: applies the hunks of a git_patch to a buffer.

This is just laying some groundwork for internal index changes
that I'm working on.

* Make GIT_INLINE an internal definition so it cannot be used in
  public headers
* Fix language in CONTRIBUTING
* Make index caps API use signed instead of unsigned values

This tweaks the pqueue_up and pqueue_down routines so that they
will not do full element swaps but instead carry over the state
of the previous loop iteration and only assign elements for which
we know the final position.  This will avoid a little bit of data
assignment which should improve performance in theory.

Also got rid of some vector helpers that I'm no longer using.

This updates the git_pqueue to simply be a set of specialized
init/insert/pop functions on a git_vector.

To preserve the pqueue feature of having a fixed size heap, I
converted the "sorted" field in git_vectors to a more general
"flags" field so that pqueue could mix in it's own flag.  This
had a bunch of ramifications because a number of places were
directly looking at the vector "sorted" field - I added a couple
new git_vector helpers (is_sorted, set_sorted) so the specific
representation of this information could be abstracted.

I accidentally wrote a separate priority queue implementation when
I was working on file rename detection as part of the file hash
signature calculation code.  To simplify licensing terms, I just
adapted that to a general purpose priority queue and replace the
old priority queue implementation that was borrowed from elsewhere.

This also removes parts of the COPYING document that no longer
apply to libgit2.

This renames git_vector_free_all to the better git_vector_free_deep
and also contains a couple of memory leak fixes based on valgrind
checks.  The fixes are specifically: failure to free global dir
path variables when not compiled with threading on and failure to
free filters from the filter registry that had not be initialized
fully.