duplicate Message-Id in issue comment notification mails (BB-8498)

These two comments:

• https://bitbucket.org/jwilk/ocrodjvu/issue/6#comment-4005334
• https://bitbucket.org/jwilk/ocrodjvu/issue/6#comment-4005511

generated notification mails with the message msg-id:

Message-ID: <issues-964630acc7914a0d4881b8f43f753ded/updated@bitbucket.org>

But RFC 5322, §3.6.4 reads
“The message identifier (msg-id) itself MUST be a globally unique identifier
for a message. The generator of the message identifier MUST guarantee that the msg-id is unique.”