feat: Change from filename to checksum as the default diff mode

Now that the checksum is a lot easier to do than before, I think it
makes more sense to pick it as the default diff mode.

It will not be as easily understandable than GNU or Git, but will show
the problematic files whilst being more available than the GNU/git diff
modes.

src/Console/Command/Diff.php

@@ -98,7 +98,7 @@ public function getConfiguration(): Configuration
                             DiffMode::values(),
                         ),
                     ),
-                    DiffMode::FILE_NAME->value,
+                    DiffMode::CHECKSUM->value,
                 ),
                 new InputOption(
                     self::CHECK_OPTION,