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.

Use the new-style index names throughout our own codebase.

Ensure that we include conflicts when calling `git_index_read_index`,
which will remove conflicts in the index that do not exist in the new
target, and will add conflicts from the new target.

Read a tree into an index, write the index, then re-open the index and
ensure that we are treesame to the original.

Read a tree into an index using `git_index_read_index` (by reading
a tree into a new index, then reading that index into the current
index), then write the index back out, ensuring that our new index
is treesame to the tree that we read.