Replace priority queue code with implementation from hashsig

Remove unused utf8 -> utf16 conversion code

Some tests of paths that can't actually be written to disk

Add flexibility to the revwalk API

commit: faster parsing

We need this from util.h and posix.h, but the latter includes common.h
which includes util.h, which means p_strlen is not defined by the time
we get to git__strndup().

Split the definition on p_strlen() off into its own header so we can use
it in util.h.

The standard library provides a very nice strnlen function, which knows
to use SSE, let's not reimplement it ourselves.

The current code issues a lot of strncmp() calls in order to check for
the end of the header, simply in order to copy it and start going
through it again. These are a lot of calls for something we can check as
we go along. Knowing the amount of parents beforehand to reduce
allocations in extreme cases does not make up for them.

Instead start parsing immediately and check for the double-newline after
each header field, leaving the raw_header allocation for the end, which
lets us go through the header once and reduces the amount of strncmp()
calls significantly.

In unscientific testing, this has reduced a shortlog-like usage (walking
though the whole history of a branch and extracting data from the
commits) of git.git from ~830ms to ~700ms and makes the time we spend in
strncmp() negligible.

Now that we no longer fail to push non-commits on a glob, let's search
on all refs when we rev-parse syntax asks us to match text.

This used to be broken, let's make sure we don't break this use-case.

Pushing a whole namespace can cause us to attempt to push non-committish
objects. Catch this situation and special-case it for ignoring this.

This shows that pusing a whole namespace can be problematic.

Let the user push committish objects and peel them to figure out which
commit to push to our queue.

This is for convenience and for allowing uses of

    git_revwalk_push_glob(w, "tags")

with annotated tags.

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 fixes a typo I made for setting the sorted flag on the index
after a reload.  That typo didn't actually cause any test failures
so I'm also adding a test that explicitly checks that the index is
correctly sorted after a reload when ignoring case and when not.

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.

Corrections for merge working directory validation

I forgot that I wrote some tests for the new priority queue code.

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.

Validating the workdir should not compare HEAD to working
directory - this is both inefficient (as it ignores the cache)
and incorrect.  If we had legitimately allowed changes in the
index (identical to the merge result) then comparing HEAD to
workdir would reject these changes as different.  Further, this
will identify files that were filtered strangely as modified,
while testing with the cache would prevent this.

Also, it's stupid slow.

Correct "new" id for reattached-HEAD reflog entry

Add `git_reference_is_note`.

Update to latest clar

Reflog completionism