rebase: generify create_autostash()

rebase: generify create_autostash()

In the future, we plan on lib-ifying create_autostash() so we need it to
be more generic. Make it more generic by making it accept a
`struct repository` argument instead of implicitly using the non-repo
functions and `the_repository`. Also, make it accept a `path` argument
so that we no longer rely have to rely on `struct rebase_options`.
Finally, make it accept a `default_reflog_action` argument so we no
longer have to rely on `DEFAULT_REFLOG_ACTION`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>