sequencer: pass explicit --no-gpg-sign to merge

sequencer: pass explicit --no-gpg-sign to merge

The merge subcommand launched for merges with non-default strategy would
use its own default behaviour to decide how to sign commits, regardless
of what opts->gpg_sign was set to. For example the --no-gpg-sign flag
given to rebase explicitly would get ignored, if commit.gpgsign was set
to true.

Fix the issue and add a test case excercising this behaviour.

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