Bug fix: fix `force` variable aliasing.

We were decrementing the `force` variable in the outer scope, with the
result that only the first directory in the outer `output` directory
was being recursed into (combined with the previous bug in the `utimes`
call which guaranteed that all output subdirectories were far in the
future).

Change-Id: I59f3b3ae66ef8909742e4f9140f5f48e528f1541