Start on fixing Submodule parent_commit annotations

These used the old Commit_ish. They may not all be the same as each
other, since they perform different validation and some will look
up a commit while others do not. In particular, this is complicated
by how Submodile.__init__ documents its parent_commit parameter: it
says to see the set_parent_commit method. But __init__ sets the
_parent_commit attribute to parent_commit, while set_parent_commit
treats its commit parameter differently, calling self.repo.commit
on it to get get a Commit object from it even if it wasn't one to
begin with. See #1869 for full details.

There may be some other subtleties as well. A number of uses of the
of the old Commit_ish type appear in git.submodule.base, all
related to parent_commit. These are all remaining references to it
(identifiable due to the rename to Old_commit_ish done in 04a2753),
though. So once the changes begun here for git.submodule.* are
done, that will also have completed the broader replacement effort
begun in 7328a00. At that point cleanup can be done in git.types
(removing Old_commit_ish introduced in 04a2753, replacing
Lit_old_commit_ish with a possibly-updated and deprecated
Lit_commit_ish to avoid a breaking change, and possibly making
further refinements to additions to the newly added docstrings).

Author: EliahKagan

git/objects/submodule/base.py

@@ -45,12 +45,13 @@
 from typing import Callable, Dict, Mapping, Sequence, TYPE_CHECKING, cast
 from typing import Any, Iterator, Union
 
-from git.types import Old_commit_ish, Literal, PathLike, TBD
+from git.types import Commit_ish, Literal, Old_commit_ish, PathLike, TBD
 
 if TYPE_CHECKING:
     from git.index import IndexFile
-    from git.repo import Repo
+    from git.objects.commit import Commit
     from git.refs import Head
+    from git.repo import Repo
 
 # -----------------------------------------------------------------------------
 
@@ -99,7 +100,7 @@ class Submodule(IndexObject, TraversableIterableObj):
     """Submodule flags. Submodules are directories with link-status."""
 
     type: Literal["submodule"] = "submodule"  # type: ignore
-    """This is a bogus type for base class compatibility."""
+    """This is a bogus type string for base class compatibility."""
 
     __slots__ = ("_parent_commit", "_url", "_branch_path", "_name", "__weakref__")
 
@@ -1242,7 +1243,7 @@ def remove(
 
         return self
 
-    def set_parent_commit(self, commit: Union[Old_commit_ish, None], check: bool = True) -> "Submodule":
+    def set_parent_commit(self, commit: Union[Commit_ish, str, None], check: bool = True) -> "Submodule":
         """Set this instance to use the given commit whose tree is supposed to
         contain the ``.gitmodules`` blob.
 
@@ -1495,7 +1496,7 @@ def url(self) -> str:
         return self._url
 
     @property
-    def parent_commit(self) -> "Old_commit_ish":
+    def parent_commit(self) -> "Commit":
         """
         :return:
             :class:`~git.objects.commit.Commit` instance with the tree containing the
@@ -1557,8 +1558,8 @@ def children(self) -> IterableList["Submodule"]:
     def iter_items(
         cls,
         repo: "Repo",
-        parent_commit: Union[Old_commit_ish, str] = "HEAD",
-        *Args: Any,
+        parent_commit: Union[Commit_ish, str] = "HEAD",
+        *args: Any,
         **kwargs: Any,
     ) -> Iterator["Submodule"]:
         """