pack.FilePackIndex: use self._file instead of file parameter

As the previous lines show, the file parameter could be unspecified. We
have already handled the case when it is unspecified, with the result in
self._file; so, use it instead of file.
Signed-off-by: Jelmer Vernooij <jelmer@samba.org>

dulwich/pack.py

@@ -346,7 +346,7 @@ class FilePackIndex(PackIndex):
         else:
             self._file = file
         if contents is None:
-            self._contents, self._size = _load_file_contents(file, size)
+            self._contents, self._size = _load_file_contents(self._file, size)
         else:
             self._contents, self._size = (contents, size)