Bug 1460487 - Set MOZ_SOURCE_CHANGESET appropriately for TB. r=dustin

Sets MOZ_SOURCE_CHANGESET to be the comm-* changeset when building
Thunderbird on Windows. This mirrors a change at line 196 that sets the
same when building for Linux and OSX.

Differential Revision: https://phabricator.services.mozilla.com/D15282

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

taskcluster/taskgraph/transforms/job/mozharness.py

@@ -287,6 +287,9 @@ def mozharness_on_generic_worker(config, job, taskdesc):
     if config.params.is_try():
         env['TRY_COMMIT_MSG'] = config.params['message'] or 'no commit message'
 
+    if run['comm-checkout']:
+        env['MOZ_SOURCE_CHANGESET'] = env['COMM_HEAD_REV']
+
     if not job['attributes']['build_platform'].startswith('win'):
         raise Exception(
             "Task generation for mozharness build jobs currently only supported on Windows"