Start of filter API + git_blob_filtered_content

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
Create public filter object and use it

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.
Make git_filter_source opaque
Add attributes to filters and fix registry

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.
Extend public filter api with filter lists

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.
Hook up filter initialize callback

I knew I forgot something
Fix longstanding valgrind warning

There was a possible circumstance that could result in reading
past the end of a buffer.  This check fixes that.
Add simple global shutdown hooks

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.
Update filter registry code

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

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.
Add ident filter

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.
Merge git_buf and git_buffer

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.
Move binary check to CRLF filter itself

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
Port tests from PR 1683

This ports over some of the tests from
    https://github.com/libgit2/libgit2/pull/1683
by @yorah and @ethomson
Rearrange clar submodule cleanup code
Make filter tests somewhat more robust

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.
Fix win32 warnings

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.
Fixing up some win32 issues with autocrlf
Fix cleanup issues with new tests
Add clar helpers for testing file equality

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.