Bug 1460487 - Update mozharness to set MOZ_SOURCE_CHANGESET for comm …

…builds. r=dustin

about:buildconfig on Thunderbird shows an incorrect mercurial source URL
that's a comm- repo, but a mozilla- changeset revision.

For Taskcluster builds at least, MOZ_SOURCE_CHANGESET is set as an environment
variable by mozharness. During the build, build/variables.py writes out
source-repo.h with a few defines, including MOZ_SOURCE_URL that gets picked
up by buildconfig.html.

Differential Revision: https://phabricator.services.mozilla.com/D7151

taskcluster/taskgraph/transforms/job/mozharness.py

@@ -182,6 +182,9 @@ def mozharness_on_docker_worker_setup(config, job, taskdesc):
     if config.params.is_try():
         env['TRY_COMMIT_MSG'] = config.params['message']
 
+    if run['comm-checkout']:
+        env['MOZ_SOURCE_CHANGESET'] = env['COMM_HEAD_REV']
+
     # if we're not keeping artifacts, set some env variables to empty values
     # that will cause the build process to skip copying the results to the
     # artifacts directory.  This will have no effect for operations that are