Consolidate str and os.PathLike[str] (use GitPython's PathLike)

Where they had been introduced and written separately in 0e1df29
before this.

The main reason is that it would have to be quoted as a string
for compatibility with Python 3.8 and lower, since os.PathLike
only defined __class_getitem__ in 3.8 and later. (The definition
in git.types already does that.) In addition, the benefit of
keeping them separate was negligible, as mentioned in 0e1df29.

Author: EliahKagan

git/diff.py

@@ -3,7 +3,6 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
-import os
 import re
 
 from git.cmd import handle_process_output
@@ -100,8 +99,8 @@ class Index:
 
     def _process_diff_args(
         self,
-        args: List[Union[str, os.PathLike[str], "Diffable", Type["Index"]]],
-    ) -> List[Union[str, os.PathLike[str], "Diffable", Type["Index"]]]:
+        args: List[Union[PathLike, "Diffable", Type["Index"]]],
+    ) -> List[Union[PathLike, "Diffable", Type["Index"]]]:
         """
         :return:
             Possibly altered version of the given args list.

git/index/base.py

@@ -645,8 +645,8 @@ def write_tree(self) -> Tree:
 
     def _process_diff_args(
         self,
-        args: List[Union[str, os.PathLike[str], "git_diff.Diffable", Type["git_diff.Diffable.Index"]]],
-    ) -> List[Union[str, os.PathLike[str], "git_diff.Diffable", Type["git_diff.Diffable.Index"]]]:
+        args: List[Union[PathLike, "git_diff.Diffable", Type["git_diff.Diffable.Index"]]],
+    ) -> List[Union[PathLike, "git_diff.Diffable", Type["git_diff.Diffable.Index"]]]:
         try:
             args.pop(args.index(self))
         except IndexError: