Don't put the message type into the list id, it breaks threading

GMail (and probably a lot of other email clients) break a thread
if the List-ID differs between the original message and any type
of reply to that original message.  This causes comments made on
a change to come up in a different thread than the original notice
message, which makes it harder to organize.

We now put the message type into an X-Gerrit-MessageType header,
in case a client wants to filter on it, and thus way we can keep
the List-ID constant throughout the "thread".

Bug: GERRIT-57
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed