git_rebase_finish relies on head_detached being set, but
rebase_init_merge was only setting it when branch->ref_name was unset.
But branch->ref_name would be set to "HEAD" in the case of detached
HEAD being either implicitly (NULL) or explicitly passed to
git_rebase_init.

Test a rebase (both a merge rebase and an inmemory rebase) with a new
commit that adds files underneath a new subfolder.

Test that we can properly abort a rebase when it is initialized by a
revspec.  This ensures that we do not conflate revspecs and refnames.

When rebasing with IDs, we do not return to the `branch`,
we remain in a detached HEAD state.

Instead of `open`ing a rebase and `abort`ing that, test that we can
`abort` a rebase that has just begun with `init`.

rebase: additional setup tests of exotic behavior

Test some additional exotic rebase setup behavior:  that we are
able to set up properly when already in a detached HEAD state,
that the caller specifies all of branch, upstream and onto,
and that the caller specifies branch, upstream and onto by ID.

When performing an in-memory rebase, keep a single index for the
duration, so that callers have the expected index lifecycle and
do not hold on to an index that is free'd out from under them.