Bug 1599643 - Add number of tasks to `mach try again --list` output r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D54842

--HG--
extra : moz-landing-system : lando

tools/tryselect/selectors/again.py

@@ -91,7 +91,12 @@ def run(index=0, purge=False, list_configs=False, list_tasks=0, message='{msg}',
     if list_configs or list_tasks > 0:
         for i, data in enumerate(history):
             msg, config = json.loads(data)
-            print('{}. {}'.format(i, msg))
+            n = len(config['tasks'])
+            print('{index}. ({n} task{s}) {msg}'.format(
+                index=i,
+                msg=msg,
+                n=n,
+                s='' if n == 1 else 's'))
 
             if list_tasks > 0:
                 indent = ' ' * 4