Use converted file path

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

git/index/util.py

@@ -38,7 +38,7 @@ class TemporaryFileSwap:
 
     def __init__(self, file_path: PathLike) -> None:
         self.file_path = os.fspath(file_path)
-        dirname, basename = osp.split(file_path)
+        dirname, basename = osp.split(self.file_path)
         fd, self.tmp_file_path = tempfile.mkstemp(prefix=basename, dir=dirname)
         os.close(fd)
         with contextlib.suppress(OSError):  # It may be that the source does not exist.