Keep the type of a Commit's parents from being too narrow

The type should be Sequence[Commit], rather than List[Commit] as
some type checkers seem already to have been inferring and as some
others were caused to infer by the indirect fix that precedes this.

Author: EliahKagan

git/objects/commit.py

@@ -79,8 +79,8 @@ class Commit(base.Object, TraversableIterableObj, Diffable, Serializable):
     # INVARIANTS
     default_encoding = "UTF-8"
 
-    # object configuration
     type: Literal["commit"] = "commit"
+
     __slots__ = (
         "tree",
         "author",
@@ -94,8 +94,11 @@ class Commit(base.Object, TraversableIterableObj, Diffable, Serializable):
         "encoding",
         "gpgsig",
     )
+
     _id_attribute_ = "hexsha"
 
+    parents: Sequence["Commit"]
+
     def __init__(
         self,
         repo: "Repo",
@@ -753,7 +756,7 @@ def _deserialize(self, stream: BytesIO) -> "Commit":
         readline = stream.readline
         self.tree = Tree(self.repo, hex_to_bin(readline().split()[1]), Tree.tree_id << 12, "")
 
-        self.parents = []  # type: List[Commit]
+        self.parents = []
         next_line = None
         while True:
             parent_line = readline()