commit: make ignore_non_trailer take buf/len

Make ignore_non_trailer take a buf/len pair instead of struct strbuf.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 8976c3d..887ccc7 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -790,7 +790,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		strbuf_stripspace(&sb, 0);
 
 	if (signoff)
-		append_signoff(&sb, ignore_non_trailer(&sb), 0);
+		append_signoff(&sb, ignore_non_trailer(sb.buf, sb.len), 0);
 
 	if (fwrite(sb.buf, 1, sb.len, s->fp) < sb.len)
 		die_errno(_("could not write commit template"));