The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

`git_oid`s now have a type, and we require the oid type when creating
the object id from creation functions.

Callers should not assume the layout of the oid structure; provide them
a macro that defines the null / zero sha1 object id.

Don't assume that a `git_oid` is a particular size; allocate
`sizeof(git_oid)` instead.

Support the ability to create a refspec that is a single object ID
without a destination.