This fixes email threading in Mail.app, that doesn't like when a thread
doesn't have stable recipients.
For instance, here is a possible sender-recipient combinations before:
From: A
To: Me
New issue
From: B
To: Me
Reply on new issue
From: A
To: Me
Another reply
Mail.app doesn't see B as a participant to the original email thread,
and decides to break the thread: it will group all messages from A
together, and separately all messages from B.
This commit makes the thread look like this:
From: A
To: gitlab/project
Cc: Me
New issue
From: B
To: gitlab/project
Cc: Me
Reply on new issue
From: A
To: gitlab/project
Cc: Me
Another reply
Mail.app sees a common recipient, and group the thread correctly.
