bug#63378: A single X-Debbugs-CC header must be used

[Arun Isaac]

> Now, I don't think the GNU Debbugs instance is kept up-to-date with
> Debbugs upstream, having its own set of changes, so I think the patch
> here should still be applied in the meantime.

I agree. And, even otherwise, it is nice for X-Debbugs-Cc to mirror the
Cc header and use a comma-separated list. So, I'm all in favour of this
patch. Specific comments about the patch itself follow in a separate
email.

I will work on a similar patch for `mumi send-email' and send it
sometime later this week. As discussed earlier, I will make `mumi
send-email' invoke `git config sendemail.headerCmd' to find out about
etc/teams.scm.