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

fixes issues with objective-git