(my $uri, my $branch) = split ' ', $value;
    $branch = defined $branch ? $branch : "default";

    (my $uri, my $id) = split ' ', $value;

    (my $res1, $stdout, $stderr) = captureStdoutStderr(600,
        ("hg", "heads", $branch));
    die "Error getting head of $branch from `$uri':\n$stderr" unless $res1;

    (my $res1, $stdout, $stderr) = captureStdoutStderr(600,("hg", "log", "-r", $id, "--template", "'{node|short} {branch}'"));
    die "Error getting branch and revision of $id from `$uri':\n$stderr" unless $res1;

    $stdout =~ m/[0-9]+:([0-9A-Fa-f]{12})/;
    my $revision = $1;
    die "Could not determine head revision of branch $branch" unless $revision;

    my ($revision, $branch) = split ' ', $stdout;