commit: encapsulate determine_whence() for sequencer

Working out which command wants to create a commit requires detailed
knowledge of the sequencer internals and that knowledge is going to
increase in subsequent commits. With that in mind lets encapsulate that
knowledge in sequencer.c rather than spreading it into builtin/commit.c.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index f5e354c..8a7c839 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -178,10 +178,7 @@ static void determine_whence(struct wt_status *s)
 {
 	if (file_exists(git_path_merge_head(the_repository)))
 		whence = FROM_MERGE;
-	else if (file_exists(git_path_cherry_pick_head(the_repository)))
-		whence = file_exists(git_path_seq_dir()) ?
-			FROM_CHERRY_PICK_MULTI : FROM_CHERRY_PICK_SINGLE;
-	else
+	else if (!sequencer_determine_whence(the_repository, &whence))
 		whence = FROM_COMMIT;
 	if (s)
 		s->whence = whence;