libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.

The more generic GIT_ERROR_SHA allows for SHA256 errors as well as SHA1.

Since users are disposing the _contents_ of the oidarray, not freeing
the oidarray itself, the proper cleanup function is
`git_oidarray_dispose`.  Deprecate `git_oidarray_free`.

`git_diff_format_email` is deprecated in favor of `git_email_create`.

buf: deprecate public git_buf writing functions

A `git_buf` is now a read-only structure as far as callers are
concerned.  This is a mechanism that we can return data to callers
using memory that is owned by the library and can be cleaned up by
callers (using `git_buf_dispose`).

A `git_buf` can no longer be allocated by callers or provided to the
library.

The signing callback should not be used; instead, callers should provide
a commit_create_cb, perform the signing and commit creation themselves.

Deprecate `git_filter_list_apply_to_data` as it takes user input as a
`git_buf`.  Users should use `git_filter_list_apply_to_buffer` instead.

`git_filter_list_stream_data` takes user input in a `git_buf`.
`git_buf` should only be used when libgit2 itself needs to allocate data
and returned to a user that they can free when they wish.  Replace it
with `git_filter_list_stream_buffer` that takes a data buffer and
length.