`exec git clone` is interrupted in tclsh

Question

I write a tcl script to retrieve remote repository using git clone

puts "start"
exec git clone https://github.com/Xilinx/device-tree-xlnx.git
puts "finished"

Here is message output

> $ tclsh ./t.tcl                                                              
start
Cloning into 'device-tree-xlnx'...
    while executing
"exec git clone git@github.com:Xilinx/device-tree-xlnx.git"
    (file "./t.tcl" line 2)

After git clone is finished, puts "finished" can't be execute.

Has anyone encountered or solved this problem?