Bugfix: Correctly clean up status bar output of the `backup` command

Due to a regression in restic 0.15.2, the status bar of the `backup` command
could leave some output behind. This happened if filenames were printed that
are wider than the current terminal width. This has now been fixed.

https://github.com/restic/restic/issues/4319
https://github.com/restic/restic/pull/4318
