sequencer: fix gpg option passed to merge subcommand

sequencer: fix gpg option passed to merge subcommand

When performing a rebase with --rebase-merges using either a custom
strategy specified with -s or an octopus merge, and at the same time
having gpgsign enabled (either rebase -S or config commit.gpgsign), the
operation would fail on making the merge commit. Instead of "-S%s" with
the key id substituted, only the bare key id would get passed to the
underlying merge command, which tried to interpret it as a ref.

Fix the issue and add test cases as suggested by Johannes Schindelin and
Junio C Hamano.

Signed-off-by: Samuel Čavoj <samuel@cavoj.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>