sequencer.c: recognize "(cherry picked from ..." as part of s-o-b footer

sequencer.c: recognize "(cherry picked from ..." as part of s-o-b footer

When 'cherry-pick -s' is used to append a signed-off-by line to a cherry
picked commit, it does not currently detect the "(cherry picked from..."
that may have been appended by a previous 'cherry-pick -x' as part of the
s-o-b footer and it will insert a blank line before appending a new s-o-b.

Let's detect "(cherry picked from...)" as part of the footer so that we
will produce this:

   Signed-off-by: A U Thor <author@example.com>
   (cherry picked from da39a3ee5e6b4b0d3255bfef95601890afd80709)
   Signed-off-by: C O Mmitter <committer@example.com>
instead of this:

   Signed-off-by: A U Thor <author@example.com>
   (cherry picked from da39a3ee5e6b4b0d3255bfef95601890afd80709)

   Signed-off-by: C O Mmitter <committer@example.com>
[with improvements from Jonathan Nieder]

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>