sequencer: plug memory leaks for the option values

sequencer: plug memory leaks for the option values

The sequencer is our attempt to lib-ify cherry-pick. Yet it behaves
like a one-shot command when it reads its configuration: memory is
allocated and released only when the command exits.

This is kind of okay for git-cherry-pick, which *is* a one-shot
command. All the work to make the sequencer its work horse was
done to allow using the functionality as a library function, though,
including proper clean-up after use.

To remedy that, take custody of the option values in question,
allocating and duping literal constants as needed and freeing them
at end.

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