fixup! BUG: Fix .groupby(categorical, sort=False) failing

Author: kernc

pandas/core/groupby.py

@@ -2315,9 +2315,11 @@ def __init__(self, index, grouper=None, obj=None, name=None, level=None,
                 # groupby
                 else:
                     cat = self.grouper.unique()
-                    cat.add_categories([c for c in self.grouper.categories
-                                        if c not in cat.categories],
-                                       inplace=True)  # GH-13179
+                    all_categories = self.grouper.categories
+                    cat.add_categories(
+                        all_categories[
+                            ~all_categories.isin(cat.categories)],
+                        inplace=True)  # GH-13179
                     self.grouper = self.grouper.reorder_categories(
                         cat.categories)