Enhancement: Improve accuracy of ETA displayed during backup

Restic's `backup` command displayed an ETA that did not adapt when the rate of
progress made during the backup changed during the course of the backup.

Restic now uses recent progress when computing the ETA. It is important to
realize that the estimate may still be wrong, because restic cannot predict
the future, but the hope is that the ETA will be more accurate in most cases.

https://github.com/restic/restic/issues/3397
https://github.com/restic/restic/pull/3563
