* rebase: Fix missing initialization of the repo pointer
While the `Rebase` structure has a pointer to the repository the rebase
is creatde in, this pointer isn't ever initialized. Fix this.
* rebase: Add wrapper for `git_rebase_inmemory_index()`
Add a new wrapper for `git_rebase_inmemory_index()`, which can be used
to retrieve the index for an in-memory rebase.
Co-authored-by: Patrick Steinhardt <psteinhardt@gitlab.com>
(cherry picked from commit e7d1b2b69fbe476c75a00cf8dcda284337facb50)
Co-authored-by: Patrick Steinhardt <ps@pks.im>