Bug 1286900 - Run Mercurial with PYTHONUNBUFFERED=1; r=me

We did the same thing for run-task in bug 1304964 because otherwise
output may get buffered, triggering "no output in N seconds"
failures in automation.

testing/mozharness/mozharness/base/vcs/mercurial.py

@@ -388,7 +388,8 @@ class MercurialVCS(ScriptMixin, LogMixin, TransferMixin):
 
         parser = RepositoryUpdateRevisionParser(config=self.config,
                                                 log_obj=self.log_obj)
-        if self.run_command(args, output_parser=parser):
+        if self.run_command(args, output_parser=parser,
+                            partial_env={'PYTHONUNBUFFERED': '1'}):
             raise VCSException('repo checkout failed!')
 
         if not parser.revision: