patchtest-send-results: improve subject line

Pull the actual email's subject line from the .mbox file and use that in
patchtest's test results response, so that it's clearer which patch it
is replying to.

(From OE-Core rev: 98ca0b151517b3544454fd5c1656a2de631c4897)

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/patchtest-send-results

@@ -53,7 +53,12 @@ with open(result_file, "r") as f:
     testresult = f.read()
 
 reply_contents = greeting + testresult + suggestions
-subject_line = f"Patchtest results for {result_basename}"
+
+# we know these patch files will only contain a single patch, so only
+# worry about the first element for getting the subject
+mbox = mailbox.mbox(args.patch)
+mbox_subject = mbox[0]['subject']
+subject_line = f"Patchtest results for {mbox_subject}"
 
 if "FAIL" in testresult:
     ses_client = boto3.client('ses', region_name='us-west-2')