Make sure thread pool has at least 1 worker.

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

git_cl.py

@@ -3073,7 +3073,7 @@ def get_cl_statuses(changes, fine_grained, max_processes=None):
       pool = ThreadPool(
           min(max_processes, len(changes_to_fetch))
               if max_processes is not None
-              else len(changes_to_fetch))
+              else max(len(changes_to_fetch), 1))
 
       fetched_cls = set()
       it = pool.imap_unordered(fetch, changes_to_fetch).__iter__()