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/Plugin/SyncMercurial.php

@@ -79,6 +79,7 @@ class IDF_Plugin_SyncMercurial
         $output = array();
         $cmd = sprintf('hg init %s', 
                        escapeshellarg($mercurial_path.'/'.$shortname));
+        $cmd = Pluf::f('idf_exec_cmd_prefix', '').$cmd;
         $ll = exec($cmd, $output, $return);
         return ($return == 0);
     }