Fix error in 'sphinx-last-updated-by-git'

Author: AA-Turner

sphinx/environment/__init__.py

@@ -131,7 +131,7 @@ def __init__(self, app: Sphinx) -> None:
         self.all_docs: dict[str, int] = {}
         # docname -> set of dependent file
         # names, relative to documentation root
-        self.dependencies: dict[str, set[_StrPath]] = {}
+        self.dependencies: dict[str, set[_StrPath]] = defaultdict(set)
         # docname -> set of included file
         # docnames included from other documents
         self.included: dict[str, set[str]] = defaultdict(set)