Bug 1286900 - Set env explicitly because mozharness; r=me

partial_env only works if the class inherits from some other base
class, which apparently not all callers of this method do. So just
pass a copy of the environ dict with PYTHONUNBUFFERED added.

MozReview-Commit-ID: Ag75x28NR4D

--HG--
extra : rebase_source : f19b2be2db0a4b321542cc353a4599481ba60146

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

@@ -388,8 +388,9 @@ class MercurialVCS(ScriptMixin, LogMixin, TransferMixin):
 
         parser = RepositoryUpdateRevisionParser(config=self.config,
                                                 log_obj=self.log_obj)
-        if self.run_command(args, output_parser=parser,
-                            partial_env={'PYTHONUNBUFFERED': '1'}):
+        env = dict(os.environ)
+        env[b'PYTHONUNBUFFERED'] = b'1'
+        if self.run_command(args, output_parser=parser, env=env):
             raise VCSException('repo checkout failed!')
 
         if not parser.revision: