Make pv more useful by passing parameters.

Disabling the non-functional progress bar and
adding average and written byte counters.

Signed-off-by: Michael Düll <michael.duell@rub.de>

btrbk

@@ -1279,7 +1279,7 @@ sub btrfs_send_receive($$$$)
     name => "btrfs send",
   };
   push @cmd_pipe, {
-    cmd => [ '/usr/bin/pv' ],
+    cmd => [ '/usr/bin/pv', '-traeb' ],
   } if($show_progress);
   push @cmd_pipe, {
     cmd => [ qw(btrfs receive), @receive_options, $target_path . '/' ],