`git_odb_open` was erroneously removed during a refactoring; add it
back.

Teach the packfile machinery to cope with SHA256.

In clar, an underscore is meaningful; avoid using it incorrectly /
unnecessarily.

Actually `cl_skip` the sha256 tests when we're not compiled for sha256,
instead of passing them.

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

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.