When doing a 'git cl archive' if there are no branches other then master the
command will crash with a 'ValueError: Number of processes must be at least 1'
when attempting to create the thread pool.

This Cl modifies the code to use the maximum of the changes and 1 as the number
of threads to create so we have at least one thread to work with.

Review-Url: https://codereview.chromium.org/2228723003