sequencer.c: teach append_signoff how to detect duplicate s-o-b

Teach append_signoff how to detect a duplicate s-o-b in the commit footer.
This is in preparation to unify the append_signoff implementations in
log-tree.c and sequencer.c.

Fixes test in t3511.

Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 1dd2ec5..7b9e2ac 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -700,7 +700,7 @@
 			previous = eol;
 		}
 
-		append_signoff(&sb, ignore_footer);
+		append_signoff(&sb, ignore_footer, 0);
 	}
 
 	if (fwrite(sb.buf, 1, sb.len, s->fp) < sb.len)