_to_relative_path to support mixing slashes and backslashes

Working on Windows you sometime end up having some paths with backslashes (windows native) and some with slashes - this PR will resolve the issue using gitpython for those kind of cases (see example below). It will also fix the issues if paths contain redundant separators or "..".

```
import git

repo = git.Repo(r"C:\gittest")
repo.index.add(r"C:\gittest\1.txt")
# Traceback (most recent call last):
#   File "c:\second_test.py", line 5, in <module>
#     repo.index.add(r"C:/gittest/2.txt")
#   File "Python311\Lib\site-packages\git\index\base.py", line 879, in add
#     paths, entries = self._preprocess_add_items(items)
#                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#   File "Python311\Lib\site-packages\git\index\base.py", line 672, in _preprocess_add_items
#     paths.append(self._to_relative_path(item))
#                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#   File "Python311\Lib\site-packages\git\index\base.py", line 657, in _to_relative_path
#     raise ValueError("Absolute path %r is not in git repository at %r" % (path, self.repo.working_tree_dir))
# ValueError: Absolute path 'C:/gittest/2.txt' is not in git repository at 'C:\\gittest'

repo.index.add(r"C:/gittest/2.txt")

repo.index.commit("test")
```

Author: Andrej730

git/index/base.py

@@ -653,7 +653,7 @@ def _to_relative_path(self, path: PathLike) -> PathLike:
             return path
         if self.repo.bare:
             raise InvalidGitRepositoryError("require non-bare repository")
-        if not str(path).startswith(str(self.repo.working_tree_dir)):
+        if not osp.normpath(str(path)).startswith(osp.normpath(str(self.repo.working_tree_dir))):
             raise ValueError("Absolute path %r is not in git repository at %r" % (path, self.repo.working_tree_dir))
         return os.path.relpath(path, self.repo.working_tree_dir)