Use upstream conversation subject for CI emails

Summary:
We have had requests to use the subject of the upstream discussion when sending
BPF CI email messages. The reason being that Gmail doesn't thread emails
properly once the subject changes [0] -- and so our indication of
failure/success and other goodies in the subject cause it to keep any CI emails
separate from the main discussion.
With this change we switch over to using the upstream subject, similar to what
bots such as kernel test robot do [1].

[0] https://support.google.com/mail/answer/5900
[1] https://lore.kernel.org/all/ZRkI4j2+dXZ9sjQ4@f61ccfac960a/

Reviewed By: chantra

Differential Revision: D51139610

fbshipit-source-id: 63aa1bc27bfc5493ded14f7c1498822186b1490a

kernel_patches_daemon/branch_worker.py

@@ -6,6 +6,9 @@
 
 import asyncio
 import copy
+import email
+import email.parser
+import email.policy
 import hashlib
 import logging
 import os
@@ -173,6 +176,15 @@ def get_github_actions_url(repo: Repository, pr: PullRequest, status: Status) ->
     return github_actions_url
 
 
+async def get_ci_email_subject(series: Series) -> str:
+    """Get the subject to use for a CI email pertaining the given series."""
+    obj = get_ci_base(series)
+    patch = await series.pw_client.get_blob(obj["mbox"])
+    parser = email.parser.BytesParser(policy=email.policy.default)
+    msg = parser.parsebytes(patch, headersonly=True)
+    return f"Re: {msg.get('subject', series.name)}"
+
+
 def furnish_ci_email_body(
     repo: Repository, pr: PullRequest, status: Status, series: Series
 ) -> str:
@@ -1001,13 +1013,12 @@ async def evaluate_ci_result(
         if status == Status.SUCCESS:
             new_label = StatusLabelSuffixes.PASS.to_label(series.version)
             not_label = StatusLabelSuffixes.FAIL.to_label(series.version)
-            subject = f"BPF CI -- success ({series.name})"
         else:
             assert status in (Status.FAILURE, Status.CONFLICT), status
             new_label = StatusLabelSuffixes.FAIL.to_label(series.version)
             not_label = StatusLabelSuffixes.PASS.to_label(series.version)
-            subject = f"BPF CI -- failure ({series.name})"
 
+        subject = await get_ci_email_subject(series)
         body = furnish_ci_email_body(self.repo, pr, status, series)
 
         labels = {label.name for label in pr.labels}