Modify annotations to accommodate NULL_TREE

This finishes the main changes being done at this time to the
annotations of diff-related methods that were started in 0e1df29.

As expected and noted in previous commits, having Diffable.diff
permit NULL_TREE entails a violation of the Liskov substitution
principle in the overridden method IndexFile.diff unless a similar
change were also made there, which could not be done correctly
without modifying its behavior to actually accept NULL_TREE (it
does not contain code to cover it, and raises ane exception if it
is passed). I am unsure if that should ultimately be done, but even
if so, it seems to me to be beyond the scope of the typing changes
being done here.

This therefore applies a suppression there. The suppression is
specific to that one parameter. The long-standing comment atop the
IndexFile.diff method, which reads as vague today, is replaced with
a specific FIXME comment describing the situation where the method
refers to the base-class docstring for documentation of its
parameters yet doesn't accept NULL_TREE (and notes the mypy error).

In effect this is really restoring and fixing the suppression that
was present before 0e1df29 rather than adding a "new" one, but at
that time the base-class parameter type was much broader since it
was a union with object as one of its alternatives, so the
situation was much less clear.

Author: EliahKagan

git/diff.py

@@ -189,7 +189,7 @@ def _process_diff_args(
 
     def diff(
         self,
-        other: Union[Literal[DiffConstants.INDEX], "Tree", "Commit", str, None] = INDEX,
+        other: Union[DiffConstants, "Tree", "Commit", str, None] = INDEX,
         paths: Union[PathLike, List[PathLike], Tuple[PathLike, ...], None] = None,
         create_patch: bool = False,
         **kwargs: Any,

git/index/base.py

@@ -1475,10 +1475,13 @@ def reset(
 
         return self
 
-    # @ default_index, breaks typing for some reason, copied into function
+    # FIXME: This is documented to accept the same parameters as Diffable.diff, but this
+    # does not handle NULL_TREE for `other`. (The suppressed mypy error is about this.)
     def diff(
         self,
-        other: Union[Literal[git_diff.DiffConstants.INDEX], "Tree", "Commit", str, None] = git_diff.INDEX,
+        other: Union[  # type: ignore[override]
+            Literal[git_diff.DiffConstants.INDEX], "Tree", "Commit", str, None
+        ] = git_diff.INDEX,
         paths: Union[PathLike, List[PathLike], Tuple[PathLike, ...], None] = None,
         create_patch: bool = False,
         **kwargs: Any,