Fix RootModule.update ignore[override] suppression

It was on the `previous_commit` parameter, and ineffective. It may
be that the original intent of the suppression was to suppress only
incompatible override type errors due to the addition of that
parameter, but there are other paramters that also mismatch by
having a different name or by being absent even though present in
the overridden base-class method. Furthermore, even coresponding
parameters mismatch in position due to the insertion of the
`previous_commit` parameter, so even if there is some way to do a
more fine-grained suppression than applying `ignore[override]` to
the entire method signature, that would be insufficient here.

This fixes one mypy error. It does so by causing it to be suppressed
effectively, not by fixing the underlying issue, which may not be
fixable since it would entail a breaking change to fix. However,
this does not introduce any new suppressions, just fixes an existing
suppression so it is effective (and probably so it operates as
intended, though maybe it is slightly stronger than intended as
discussed above).

Author: EliahKagan

Diff for: git/objects/submodule/root.py

@@ -75,9 +75,9 @@ def _clear_cache(self) -> None:
 
     # { Interface
 
-    def update(
+    def update(  # type: ignore[override]
         self,
-        previous_commit: Union[Commit_ish, None] = None,  # type: ignore[override]
+        previous_commit: Union[Commit_ish, None] = None,
         recursive: bool = True,
         force_remove: bool = False,
         init: bool = True,