title: `exec git clone` is interrupted in tclsh
body: <p>I write a tcl script to retrieve remote repository using <code>git clone</code></p>
  <pre><code>puts &quot;start&quot;
  exec git clone https://github.com/Xilinx/device-tree-xlnx.git
  puts &quot;finished&quot;
  </code></pre>
  <p>Here is message output</p>
  <pre><code>&gt; $ tclsh ./t.tcl                                                              
  start
  Cloning into 'device-tree-xlnx'...
      while executing
  &quot;exec git clone git@github.com:Xilinx/device-tree-xlnx.git&quot;
      (file &quot;./t.tcl&quot; line 2)
  </code></pre>
  <p>After <code>git clone</code> is finished, <code>puts &quot;finished&quot;</code> can't be execute.</p>
  <p>Has anyone encountered or solved this problem?</p>