Also set References: header for CI emails

Summary:
Let's also set the References: header in emails in case some email clients
decide not to use In-Reply-To: for threading matters (as seems to be allowed by
the "spec").

Reviewed By: chantra

Differential Revision: D50231525

fbshipit-source-id: 4470fdd498b021b6349fad8a3231d7757f38dc23

kernel_patches_daemon/branch_worker.py

@@ -252,6 +252,7 @@ async def send_email(config: EmailConfig, series: Series, subject: str, body: st
         msg["In-Reply-To"] = f"{series.cover_letter['msgid']}"
     else:
         msg["In-Reply-To"] = f"{series.patches[0]['msgid']}"
+    msg["References"] = msg["In-Reply-To"]
     msg["Subject"] = subject
     msg["From"] = config.smtp_from
     if to_list: