sequencer: assign only free()able strings to gpg_sign

sequencer: assign only free()able strings to gpg_sign

The gpg_sign member of the replay_opts structure is of type `char *`,
meaning that the sequencer deems the string to which gpg_sign points to
be under its custody, i.e. it needs to be free()d by the sequencer.

Therefore, let's only assign malloc()ed buffers to it.

Reported-by: Kaartic Sivaraam <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>