Fix a format string bug in "cat -n".

MANIFEST

@@ -127,8 +127,10 @@ MANIFEST.SKIP
 META.yml
 README.pod
 t/compile.t
+t/data/cat/cat-n-1.txt
 t/data/sort/ints1.txt
 t/data/sort/letters1.txt
+t/cat.t
 t/echo.t
 t/factor.t
 t/false.t

bin/cat

@@ -63,8 +63,8 @@ while (<>) {
         $was_empty = $is_empty;
     }
 
-    $_ = sprintf "%6d  $_", ++ $count if $number_lines ||
-                                         $number_non_blanks && /\S/;
+    $_ = sprintf "%6d  %s", ++ $count, $_ if $number_lines ||
+                                             $number_non_blanks && /\S/;
 
     $_ =~ s/$/\$/   if $ends;
     if ($nonprinting) {

t/cat.t

@@ -0,0 +1,43 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+
+use Test::More tests => 1;
+
+sub _lines2re
+{
+    return join( qq#\r?\n#, @_ ) . qq#\r?\n?#;
+}
+
+sub test_cat
+{
+    local $Test::Builder::Level = $Test::Builder::Level + 1;
+    my ($args) = @_;
+
+    my $re = _lines2re( @{ $args->{lines} } );
+    return like(
+        scalar(`$^X -Ilib bin/cat @{$args->{flags}} @{$args->{files}}`),
+        qr#\A$re\z#ms, $args->{blurb} );
+}
+
+# TEST
+test_cat(
+    {
+        blurb => "format string expansion in cat -n",
+        files => [qw( t/data/cat/cat-n-1.txt )],
+        flags => [qw( -n )],
+        lines => ["     1  %d"],
+    }
+);
+
+__END__
+
+=head1 COPYRIGHT & LICENSE
+
+Copyright 2018 by Shlomi Fish
+
+This code is licensed under the Artistic License 2.0
+L<https://opensource.org/licenses/Artistic-2.0>, or at your option any later
+version of the Artistic License from TPF ( L<https://www.perlfoundation.org/> )
+.

t/data/cat/cat-n-1.txt

@@ -0,0 +1 @@
+%d