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

Fix cl_assert_equal_sz error message formating

SSH: Clone fails with errors: ERROR: Repository invalid & Early EOF

errors: Fix format of some error messages

revwalk: allow simplifying by first-parent

Make odb_loose return EAMBIGUOUS when required

When enabled, only the first parent of each commit will be queued,
enabling a simple way of using first-parent simplification.

fixing headers with bad values for objective-c