Add git_commit_amend API

This adds an API to amend an existing commit, basically a shorthand
for creating a new commit filling in missing parameters from the
values of an existing commit.  As part of this, I also added a new
"sys" API to create a commit using a callback to get the parents.
This allowed me to rewrite all the other commit creation APIs so
that temporary allocations are no longer needed.

More reflogness

refs: conditional ref updates

This fixes a number of warnings with the Windows 64-bit build
including a test failure in test_repo_message__message where an
invalid pointer to a git_buf was being used.

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.

Add it under the git_reference_remove() name, letting the user pass the
repo and name, analogous to unconditional setting/creation.

If the type of the on-disk reference has changed, the old value
comparison should fail.

Recognize when the reference has changed since we loaded it.

We don't actually pass the old value yet.

We will reuse this later for deletion.

Bring the race detection goodness to symbolic references as well.

Add a parameter to the backend to allow checking for the old symbolic
target.

Free the old ref even on success.

In case we loose the race to update the reference, return GIT_EMODIFIED
to let the user distinguish it from other types of errors.