Apply the locale encoding to `tar --version`

Fixes a bug with non-UTF8 locales introduced by
2b816075dc.

CHANGELOG

@@ -10,6 +10,7 @@ v 6.9.0
   - Stop refreshing comments when the tab is hidden
   - Improve issue and merge request mobile UI (Drew Blessing)
   - Document how to convert a backup to PostgreSQL
+  - Fix locale bug in backup manager
 
 v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion

lib/backup/manager.rb

@@ -101,7 +101,7 @@ module Backup
 
     def tar_version
       tar_version, _ = Gitlab::Popen.popen(%W(tar --version))
-      tar_version.split("\n").first
+      tar_version.force_encoding('locale').split("\n").first
     end
   end
 end