Bug 1462462 - Log mozharness command environment only when it changes; r=jmaher

If the environment used for a mozharness command is identical to the previous
command's environment, report only "same as previous command".

testing/mozharness/mozharness/base/script.py

@@ -1382,8 +1382,12 @@ class ScriptMixin(PlatformMixin):
             if partial_env:
                 self.info("Using partial env: %s" % pprint.pformat(partial_env))
                 env = self.query_env(partial_env=partial_env)
+        else:
+            if hasattr(self, 'previous_env') and env == self.previous_env:
+                self.info("Using env: (same as previous command)")
             else:
                 self.info("Using env: %s" % pprint.pformat(env))
+                self.previous_env = env
 
         if output_parser is None:
             parser = OutputParser(config=self.config, log_obj=self.log_obj,