patman: Suppress duplicate signoffs only for real patches

There is an unfortunate bug in the signoff suppression logic. The first
pass is performed with 'git log', and all signoffs are added to the
supression set, such that the second time (when processing the real
patches) we always suppress the signoffs.

Correct this by only suppressing signoffs in the second pass.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Michal Simek <monstr@monstr.eu>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com>