Make all one-element __slots__ be tuples

Most already were, but a few were strings.

git/index/typ.py

@@ -40,7 +40,7 @@ class BlobFilter:
     The given paths are given relative to the repository.
     """
 
-    __slots__ = "paths"
+    __slots__ = ("paths",)
 
     def __init__(self, paths: Sequence[PathLike]) -> None:
         """

git/objects/tree.py

@@ -109,7 +109,7 @@ class TreeModifier:
     the cache of a tree, will be sorted. This ensures it will be in a serializable state.
     """
 
-    __slots__ = "_cache"
+    __slots__ = ("_cache",)
 
     def __init__(self, cache: List[TreeCacheTup]) -> None:
         self._cache = cache
@@ -214,7 +214,8 @@ class Tree(IndexObject, git_diff.Diffable, util.Traversable, util.Serializable):
     """
 
     type: Literal["tree"] = "tree"
-    __slots__ = "_cache"
+
+    __slots__ = ("_cache",)
 
     # Actual integer IDs for comparison.
     commit_id = 0o16  # Equals stat.S_IFDIR | stat.S_IFLNK - a directory link.

git/util.py

@@ -713,7 +713,7 @@ def update(
 class CallableRemoteProgress(RemoteProgress):
     """An implementation forwarding updates to any callable."""
 
-    __slots__ = "_callable"
+    __slots__ = ("_callable",)
 
     def __init__(self, fn: Callable) -> None:
         self._callable = fn