Make sure pre and post merge return strings

Author: bircher

src/PhpMerge/GitMerge.php

@@ -293,7 +293,7 @@ protected static function getConflicts($file, $baseText, $remoteText, $localText
      * @param $text
      * @return string
      */
-    protected static function preMergeAlter($text)
+    protected static function preMergeAlter($text) : string
     {
         // Append new lines so that conflicts at the end of the text work.
         return $text . "\nthe\nend";
@@ -303,10 +303,10 @@ protected static function preMergeAlter($text)
      * @param $text
      * @return bool|string
      */
-    protected static function postMergeAlter($text)
+    protected static function postMergeAlter($text) : string
     {
         // Remove the appended lines.
-        return substr($text, 0, -8);
+        return (string) substr($text, 0, -8);
     }
 
     /**