sequencer: use memoized sequencer directory path

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 1cba3b7..9fddb19 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -183,7 +183,7 @@ static void determine_whence(struct wt_status *s)
 		whence = FROM_MERGE;
 	else if (file_exists(git_path_cherry_pick_head())) {
 		whence = FROM_CHERRY_PICK;
-		if (file_exists(git_path(SEQ_DIR)))
+		if (file_exists(git_path_seq_dir()))
 			sequencer_in_use = 1;
 	}
 	else