Previous change was wrong... We want to update to a specific version, so hg pull -u won't work.

src/Composer/Downloader/HgDownloader.php

@@ -51,7 +51,7 @@ class HgDownloader implements DownloaderInterface
         }
 
         $this->enforceCleanDirectory($path);
-        system(sprintf('cd %s && hg pull -u %s', $path, $target->getSourceReference()));
+        system(sprintf('cd %s && hg pull && hg up %s', $path, escapeshellarg($target->getSourceReference())));
     }
 
     /**