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.
Introduce the ability to rebase in-memory or in a bare repository.
When `rebase_options.inmemory` is specified, the resultant `git_rebase`
session will not be persisted to disk. Callers may still analyze
the rebase operations, resolve any conflicts against the in-memory
index and create the commits. Neither `HEAD` nor the working
directory will be updated during this process.
