Backport PR #9943: Only track unique history of commands

Right now `last_cell` will always be an empty string. The intended use is to track the last command to only track a unique history.

Also removed some trailing whitespace 😄

Related to  #9683.

IPython/terminal/interactiveshell.py

@@ -230,6 +230,7 b' class TerminalInteractiveShell(InteractiveShell):'
             cell = cell.rstrip()
             if cell and (cell != last_cell):
                 history.append(cell)
+                last_cell = cell
 
         self._style = self._make_style_from_name_or_cls(self.highlighting_style)
         style = DynamicStyle(lambda: self._style)