Add an example replacing multiple lines

Author: fredemmott

tests/StringDiffTest.php

@@ -61,10 +61,10 @@ public function testUnifiedDiff(string $name): void {
     expect($diff)->toBeSame(
       \file_get_contents($base.'.udiff.expect'),
       'Did not match expected contents '.
-      '(from diff -u %s %s | tail -n +3 > %s.udiff.expect)',
-      $base,
-      $base,
-      $base,
+      '(from diff -u %s.a %s.b | tail -n +3 > %s.udiff.expect)',
+      $name,
+      $name,
+      $name,
     );
   }
 }

tests/examples/replace_multi.a

@@ -0,0 +1,3 @@
+foo
+bar
+baz

tests/examples/replace_multi.b

@@ -0,0 +1,3 @@
+foo
+herp
+derp

tests/examples/replace_multi.udiff.expect

@@ -0,0 +1,6 @@
+@@ -1,3 +1,3 @@
+ foo
+-bar
+-baz
++herp
++derp