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 gitpython-developers#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.base 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

Diff for: 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"]:
         """