This contains a few bug fixes and some header and API cleanups.

The main API change is that filters should now use GIT_PASSTHROUGH
to indicate that they wish to skip processing a file instead of
GIT_ENOTFOUND.

The bug fixes include a possible out-of-range buffer access in
the ident filter, a filter ordering problem I introduced into the
custom filter tests on Windows, and a filter buf NUL termination
issue that was coming up on Linux.

This adds more tests of filters, including the ident filter when
mixed with custom filters.  I was able to combine with the reverse
filter and demonstrate that the order of filter application with
the default priority constants matches the order of core Git.

Also, this fixes two issues in the ident filter: preventing ident
expansion on binary files and avoiding a NULL dereference when
dollar sign characters are found without Id.

Fixed the filter order to match core Git, too.

This test demonstrates an interesting behavior of core Git (which
is totally reasonable and which libgit2 matches, although mostly
by coincidence).  If you use the ident filter and commit a file
with a garbage ident in it, like '$Id: this is just garbage$' and
then immediately do a 'git checkout-index' with the new file, Git
will not consider the file out of date and will not overwrite the
file with an updated $Id$.  Libgit2 has the same behavior.  If you
remove the file and then do a checkout-index, it will be replaced
with a filtered version that has injected the OID correctly.

There were a lot of places in the test code base that were creating
a commit from the index on the current branch.  This just adds a
helper to handle that case pretty easily.  There was only one test
where this change ended up tweaking the test data, so pretty easy
and mostly just a cleanup.

These are a couple of new clar helpers for testing that a file
has expected contents that I extracted from the checkout code.

Actually wrote this as part of an abandoned earlier attempt at a
new filters API, but it will be useful now for some of the tests
I'm going to write.

I wish MSVC understood that "const char **" is not a const ptr,
but it a non-const pointer to an array of const ptrs.  Does that
seem like too much to ask.

The global and system config could interfere with the filter
tests by imposing CRLF filtering where it was not anticipated.
This better isolates the tests from the system settings.

This ports over some of the tests from
    https://github.com/libgit2/libgit2/pull/1683
by @yorah and @ethomson

Checkout should not reject binary files from filters, as a filter
may actually wish to operate on binary files.  The CRLF filter should
reject binary files itself if it wishes to.  Moreover, the CRLF
filter requires this logic so that users can emulate the checkout
data in their odb -> workdir filtering.

Conflicts:
	src/checkout.c
	src/crlf.c

This makes the git_buf struct that was used internally into an
externally available structure and eliminates the git_buffer.

As part of that, some of the special cases that arose with the
externally used git_buffer were blended into the git_buf, such as
being careful about git_buf objects that may have a NULL ptr and
allowing for bufs with a valid ptr and size but zero asize as a
way of referring to externally owned data.

This adds the ident filter (that knows how to replace $Id$) and
tweaks the filter APIs and code so that git_filter_source objects
actually have the updated OID of the object being filtered when
it is a known value.

Extend the git2/sys/filter API with functions to look up a filter
and add it manually to a filter list.  This requires some trickery
because the regular attribute lookups and checks are bypassed when
this happens, but in the right hands, it will allow a user to have
granular control over applying filters.

This updates the git filter registry to be a little cleaner and
plugs some memory leaks.

Increasingly there are a number of components that want to do some
cleanup at global shutdown time (at least if there are not going
to be memory leaks).  This creates a very simple system of shutdown
hooks that will be invoked by git_threads_shutdown.  Right now, the
maximum number of hooks is hardcoded, but since adding a hook is
not a public API, it should be fine and I thought it was better to
start off with really simple code.

There was a possible circumstance that could result in reading
past the end of a buffer.  This check fixes that.

I knew I forgot something

This moves the git_filter_list into the public API so that users
can create, apply, and dispose of filter lists.  This allows more
granular application of filters to user data outside of libgit2
internals.

This also converts all the internal usage of filters to the public
APIs along with a few small tweaks to make it easier to use the
public git_buffer stuff alongside the internal git_buf.

The filter registry as implemented was too primitive to actually
work once multiple filters were coming into play.  This expands
the implementation of the registry to handle multiple prioritized
filters correctly.

Additionally, this adds an "attributes" field to a filter that
makes it really really easy to implement filters that are based
on one or more attribute values.  The lookup and even simple value
checking can all happen automatically without custom filter code.

Lastly, with the registry improvements, this fills out the filter
lifecycle callbacks, with initialize and shutdown callbacks that
will be called before the filter is first used and after it is
last invoked.  This allows for system-wide initialization and
cleanup by the filter.

This creates include/sys/filter.h with a basic definition of a
git_filter and then converts the internal code to use it.  There
are related internal objects (git_filter_list) that we will want
to publish at some point, but this is a first step.

This begins the process of exposing git_filter objects to the
public API.  This includes:

* new public type and API for `git_buffer` through which an
  allocated buffer can be passed to the user
* new API `git_blob_filtered_content`
* make the git_filter type and GIT_FILTER_TO_... constants public

Support config includes

Fix failure in win32 git path guessing

Demand read only access to registry key instead of full access.
This might happen in Windows Vista and later.

The buffer size 0 was definitely not enough so it failed

No such thing as an orphan branch

Provide libssh2 error message

Unfortunately git-core uses the term "unborn branch" and "orphan
branch" interchangeably. However, "orphan" is only really there for
the checkout command, which has the `--orphan` option so it doesn't
actually create the branch.

Branches never have parents, so the distinction of a branch with no
parents is odd to begin with. Crucially, the error messages deal with
unborn branches, so let's use that.

git_clone supports init_options

This tests the fixes for issues from #1849

Treewalker

Prevent git_tree_walk 'skip entry' callback return code from leaking through as the return value of git_tree_walk