scripts: dbgmtree.py: Fixed minor mtree rendering/traversal issues

- Added TreeArt __bool__ and __len__.

  This was causing a crash in _treeartfrommtreertree when rtree was
  empty.

  The code was not updated in the set -> TreeArt class transition, and
  went unnoticed because it's unlikely to be hit unless the filesystem
  is corrupt.

  Fortunately(?) realtime rendering creates a bunch of transiently
  corrupt filesystem images.

- Tweaked lookupleaf to not include mroots in their own paths.

  This matches the behavior of leaf mdirs, and is intentionally
  different from btree's lookupleaf which needs to lookup the leaf rattr
  to terminate.

- Tweaked leaves to not remove the last path entry if it is an mdir.

  This hid the previous lookupleaf inconsistency. We only remove the
  last rbyd from the path because it is redundant, and for mdirs/mroots
  it should never be redundant.

  I ended up just replacing the corrupt check with an explicit check
  that the rbyd is redundant. This should be more precise and avoid
  issues like this in the future.

  Also adopted explicit redundant checks in Btree.leaves and
  Lfs.File.leaves.

scripts/dbgrbyd.py

@@ -1299,6 +1299,15 @@ class TreeArt:
         else:
             self.width = 0
 
+    def __iter__(self):
+        return iter(self.tree)
+
+    def __bool__(self):
+        return bool(self.tree)
+
+    def __len__(self):
+        return len(self.tree)
+
     # render an rbyd rbyd tree for debugging
     @classmethod
     def _fromrbydrtree(cls, rbyd, **args):