post-receive

#!/usr/bin/perl

use strict;
use warnings;

my $mirror_branch = '__mirror';

for (<>) {
  chomp;
  my ($old, $new, $branch) = split / /;
  if ($branch eq "refs/remotes/$mirror_branch") {
    system(qw[git --work-tree=.. checkout], $new);
    last;
  }
}