sequencer: restrict scope of a formerly public function

The function to add the `exec` commands to the todo list only needed to
be public API because it was not only used internally by the sequencer,
but also by `git rebase --preserve-merges`.

Now that that mode has been removed, we no longer need that function to
be scoped publicly.

Helped-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -5387,8 +5387,8 @@ int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
  * Add commands after pick and (series of) squash/fixup commands
  * in the todo list.
  */
-void todo_list_add_exec_commands(struct todo_list *todo_list,
-				 struct string_list *commands)
+static void todo_list_add_exec_commands(struct todo_list *todo_list,
+					struct string_list *commands)
 {
 	struct strbuf *buf = &todo_list->buf;
 	size_t base_offset = buf->len;

sequencer.h

@@ -160,8 +160,6 @@ int sequencer_remove_state(struct replay_opts *opts);
 int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 			  const char **argv, unsigned flags);
 
-void todo_list_add_exec_commands(struct todo_list *todo_list,
-				 struct string_list *commands);
 int complete_action(struct repository *r, struct replay_opts *opts, unsigned flags,
 		    const char *shortrevisions, const char *onto_name,
 		    struct commit *onto, const struct object_id *orig_head,