Possible threading issues in nm 0.32

[Michael J Gruber <git@grubix.eu>]

Hi there

After updating to notmuch 0.32 I noticed a few strange threadings that
went away after downgrading to 0.31.3 and reindexing. I can't pin-point
that yet but wanted to give an early warning so you can look out for
that, too.

My setup: mail is fetched with mbsync and indexed with "notmuch new"
(with afew run from pre and post hooks in move and tag mode, using the
notmuch bindings).

"Strange threadings": Unrelated mails without relevant headers
(references, in-reply-to) from different folders end up in the same
thread with nm 0.32; reindexing the thread with nm 0.31 puts them in
different threads as expected.

I am using a mix of file and db config as is usual pre 0.32 but that
should not affect threading, I think.

Michael