Enhancement: Do not rewrite unchanged lines every frame in status bar

Status bars were entirely rewritten every frame if any of its content has updated.
This behavior had made any user interaction (such as selection) with status bar
impossible in certain terminal emulators, even with the unchanged lines.
Now it writes changed lines only at status bar update, thereby improving 
user experience in those terminal emulators.

https://github.com/restic/restic/issues/5562
https://github.com/restic/restic/pull/5648