sequencer: use static initializers for replay_opts

sequencer: use static initializers for replay_opts

This change is not completely faithful: instead of initializing all fields
to 0, we choose to initialize command and subcommand to -1 (instead of
defaulting to REPLAY_REVERT and REPLAY_NONE, respectively). Practically,
it makes no difference at all, but future-proofs the code to require
explicit assignments for both fields.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>