Fix Diff::getText() when $end is null

Signed-off-by: Jack Cherng <jfcherng@gmail.com>

Author: jfcherng

src/Diff.php

@@ -226,14 +226,12 @@ public function render(AbstractRenderer $renderer): string
      *
      * @param string[] $lines the array of lines
      * @param int      $start the starting number
-     * @param null|int $end   the ending number. If not supplied, only the item in $start will be returned.
+     * @param null|int $end   the ending number. If not supplied, only the item in $start will be sliced.
      *
      * @return string[] array of all of the lines between the specified range
      */
     private function getText(array $lines, int $start = 0, ?int $end = null): array
     {
-        return $start === 0 && (!isset($end) || $end === \count($lines))
-            ? $lines
-            : \array_slice($lines, $start, isset($end) ? $end - $start : 1);
+        return \array_slice($lines, $start, ($end ?? $start + 1) - $start);
     }
 }