HgDriver: don't run command in non-existing directory

src/Composer/Repository/Vcs/HgDriver.php

@@ -71,7 +71,7 @@ class HgDriver extends VcsDriver
                     return sprintf('hg clone --noupdate %s %s', ProcessExecutor::escape($url), ProcessExecutor::escape($repoDir));
                 };
 
-                $hgUtils->runCommand($command, $this->url, $this->repoDir);
+                $hgUtils->runCommand($command, $this->url, null);
             }
         }