Bugfix: Improve reliability of restoring large files

In some cases restic failed to restore large files that frequently contain the
same file chunk. In combination with certain backends, this could result in
network connection timeouts that caused incomplete restores.

Restic now includes special handling for such file chunks to ensure reliable
restores.

https://github.com/restic/restic/pull/4626
https://forum.restic.net/t/errors-restoring-with-restic-on-windows-server-s3/6943
