builtin/rebase: do not assign default backend to non-constant field

builtin/rebase: do not assign default backend to non-constant field

The `struct rebase_options::default_backend` field is a non-constant
string, but is being assigned a constant via `REBASE_OPTIONS_INIT`.
Fix this by using `xstrdup()` to assign the variable and introduce a new
function `rebase_options_release()` that releases memory held by the
structure, including the newly-allocated variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>