builtin/rebase: fix leaking `commit.gpgsign` value

builtin/rebase: fix leaking `commit.gpgsign` value

In `get_replay_opts()`, we override the `gpg_sign` field that already
got populated by `sequencer_init_config()` in case the user has
"commit.gpgsign" set in their config. This creates a memory leak because
we overwrite the previously assigned value, which may have already
pointed to an allocated string.

Let's plug the memory leak by freeing the value before we overwrite it.

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