Fixed issue 119, no environment for the shell_exec and exec calls.

For each call exec or shell_exec, I have added the ability to prepend a
string. For example '/usr/bin/env -i '.

src/IDF/Scm/Mercurial.php

@@ -398,7 +398,8 @@ class IDF_Scm_Mercurial
      */
     public function getArchiveCommand($commit, $prefix='')
     {
-        return sprintf('hg archive --type=zip -R %s -r %s -',
+        return sprintf(Pluf::f('idf_exec_cmd_prefix', '').
+                       'hg archive --type=zip -R %s -r %s -',
                        escapeshellarg($this->repo),
                        escapeshellarg($commit));
     }