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.

clang's docparser highlighted these.

The original libpqueue file were licensed under Apache 2.0 so
therefore should retain their copyrights and header as per the
license terms at http://www.apache.org/licenses/LICENSE-2.0

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

There were quite a few places were spaces were being used instead of
tabs. Try to catch them all. This should hopefully not break anything.
Except for `git blame`. Oh well.

1. The license header is technically not valid if it doesn't have a
copyright signature.

2. The COPYING file has been updated with the different licenses used in
the project.

3. The full GPLv2 header in each file annoys me.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>

The `reset` call now removes the pushed commits so we can reuse
the revwalker. The API documentation has been updated with the details.