Bugfix: Fix terminal output redirection for PowerShell

When redirecting the output of restic using PowerShell on Windows, the
output contained terminal escape characters. This has been fixed by
properly detecting the terminal type.

In addition, the mintty terminal now shows progress output for the backup
command.

https://github.com/restic/restic/issues/3111
https://github.com/restic/restic/pull/3325
