diff: simplify LCS_matrix() (#915)

* Initialise $x[$i] to new listref [0] in one statement, instead of assigning list element 0 separately
* Calling standard max() function avoids temporary variables when compared items are different
* Write loops in idiomatic form with ranges
* This was intended as a readability change, but I observed that when generating diffs on a large text files the performance is slightly improved

%time perl diff.old -u bc bc.new > /dev/null
real	0m11.003s
...
real	0m10.845s
...
real	0m10.965s

%time perl diff.new -u bc bc.new > /dev/null
real	0m9.470s
...
real	0m9.546s
...
real	0m9.329s

bin/diff

@@ -609,6 +609,8 @@ package
 
 use strict;
 
+use List::Util qw(max);
+
 our $VERSION = '0.57';
 
 sub LCS_matrix {
@@ -626,23 +628,18 @@ sub LCS_matrix {
   $al = @$a;
   $bl = @$b;
 
-  my ($i, $j);
-
   $x[0] = [(0) x ($bl+1)];
-  for ($i=1; $i<=$al; $i++) {
-    my $r = $x[$i] = [];
-    $r->[0] = 0;
-    for ($j=1; $j<=$bl; $j++) {
+  for my $i (1 .. $al) {
+    my $r = $x[$i] = [ 0 ];
+    for my $j (1 .. $bl) {
       # If the first two items are the same...
       if (defined $eq
 	  ? $eq->($a->[-$i], $b->[-$j])
 	  : $a->[-$i] eq $b->[-$j]
 	 ) {
 	$r->[$j] = 1 + $x[$i-1][$j-1];
       } else {
-	my $pi = $x[$i][$j-1];
-	my $pj = $x[$i-1][$j];
-	$r->[$j] = ($pi > $pj ? $pi : $pj);
+	$r->[$j] = max($x[$i][$j-1], $x[$i-1][$j]);
       }
     }
   }