Fix missing References from broken muas.

[Arvid Picciani]

Most of my mail comes from the 50MLs i'm subscribed to. Unfortunately
some MUAs suck that much, they don't even respond in threads.
My idea how to fix them would be:

   $ notmuch search subject:Foo and from douche at brokenwebmua.com
   thread:00000000000003ff  [..] douche at brokenwebmua.com,me Foo
   thread:00000000000003cb  [..] douche at brokenwebmua.com,me Re:Foo
   thread:00000000000003cd  [..] douche at brokenwebmua.com,me Re:Foo
   thread:00000000000003c7  [..] douche at brokenwebmua.com,me Re:Foo

   $ notmuch thread-align subject:Foo and from:douche at brokenwebmua.com
   aligned 4 threads

   $ notmuch search subject:Foo and from douche at brokenwebmua.com
   thread:00000000000003dd  [..]  Foo
   

That of course makes the optimistic assumption, that the douche in
question did actually change the subject when he changed the topic, 
and has the date: field set properly. At least in my case that's true.

Thoughts?