Users should provide us an array of object ids; we don't need a separate
type. And especially, we should not be mutating user-providing values.
Instead, use `git_oid *` in the shallow code.

If a user attempts to add a custom extension that the system already
supports, or that is already in their list of custom extensions, de-dup
it.

stash: partial stash specific files

Teach the repository about the `objectformat` extension, supporting
`sha1` always and `sha256` when the experimental sha256 support is
active.

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.

Tidy up `nfmt` / `pathfmt`.