playlist: let playlist-prev go to last item in playlist

Previously, playlist-prev didn't work if you played a playlist to
completion using --idle and tried to go back. Naturally, one would
expect this to bring up the last item in the playlist, but nothing
happens. This is just because playlist_get_next is stupid and doesn't
take this into account since pl->current is NULL and thus returns NULL.
Fix this by considering the direction, checking if the playlist was
played to completion and grabbing the last entry in the index.

common/playlist.c

@@ -205,6 +205,8 @@ struct playlist_entry *playlist_get_last(struct playlist *pl)
 struct playlist_entry *playlist_get_next(struct playlist *pl, int direction)
 {
     assert(direction == -1 || direction == +1);
+    if (!pl->current && pl->playlist_completed && direction < 0)
+        return playlist_entry_from_index(pl, pl->num_entries - 1);
     if (!pl->current)
         return NULL;
     assert(pl->current->pl == pl);

common/playlist.h

@@ -70,6 +70,7 @@ struct playlist {
     // current_was_replaced is set to true.
     struct playlist_entry *current;
     bool current_was_replaced;
+    bool playlist_completed;
 
     uint64_t id_alloc;
 };

player/loadfile.c

@@ -1752,6 +1752,7 @@ static void play_current_file(struct MPContext *mpctx)
 
     mpctx->playback_initialized = true;
     mpctx->playing->playlist_prev_attempt = false;
+    mpctx->playlist->playlist_completed = false;
     mp_notify(mpctx, MPV_EVENT_FILE_LOADED, NULL);
     update_screensaver_state(mpctx);
     clear_playlist_paths(mpctx);
@@ -1992,6 +1993,9 @@ void mp_play_files(struct MPContext *mpctx)
             new_entry = mpctx->playlist->current;
         }
 
+        if (!new_entry)
+            mpctx->playlist->playlist_completed = true;
+
         mpctx->playlist->current = new_entry;
         mpctx->playlist->current_was_replaced = false;
         mpctx->stop_play = new_entry ? PT_NEXT_ENTRY : PT_STOP;