sequencer: update `done_nr' when skipping commands in a todo list

In a todo list, `done_nr' is the number of commands that were executed
or skipped, but skip_unnecessary_picks() did not update it.

This variable is mostly used by command prompts (ie. git-prompt.sh and
the like).  As in the previous commit, this inconsistent behaviour is
not a problem yet, but it would start to matter at the end of this
series the same reason.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -5054,6 +5054,7 @@ static int skip_unnecessary_picks(struct repository *r,
 		MOVE_ARRAY(todo_list->items, todo_list->items + i, todo_list->nr - i);
 		todo_list->nr -= i;
 		todo_list->current = 0;
+		todo_list->done_nr += i;
 
 		if (is_fixup(peek_command(todo_list, 0)))
 			record_in_rewritten(base_oid, peek_command(todo_list, 0));