Adds initial implementation of --dirty option.

src/Support/DirtyTestCaseFilter.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pest\Support;
+
+use Pest\Contracts\TestCaseFilter;
+use Symfony\Component\Process\Process;
+
+final class DirtyTestCaseFilter implements TestCaseFilter
+{
+    /**
+     * @var array<string>
+     */
+    private array $changedFiles = [];
+
+    public function __construct(private string $projectRoot)
+    {
+        $this->loadDiff();
+    }
+
+    public function canLoad(string $suiteClassFile): bool
+    {
+        $relativePath = str_replace($this->projectRoot, '', $suiteClassFile);
+
+        if (str_starts_with($relativePath, '/')) {
+            $relativePath = substr($relativePath, 1);
+        }
+
+        return in_array($relativePath, $this->changedFiles, true);
+    }
+
+    private function loadDiff(): void
+    {
+        $process = new Process([
+            'git',
+            'diff',
+            '--name-only',
+            'HEAD',
+            '--',
+            '*.php',
+        ]);
+        $process->run();
+
+        $this->changedFiles = explode(PHP_EOL, trim($process->getOutput()));
+    }
+}