Ensure that when all files beneath a directory are ignored that we
add the files when FORCE is specified.

Ensure that when all files beneath a directory are ignored that we
actually ignore the files.

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.

Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.