Update all cc code using .clang-format; please do so as well for future PRs
etc.; emacs has a handy 'clang-format' mode to make this automatic.
For comparing old changes with git blame, we can disregard this one using
--ignore-rev
(see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame )
Not only check for duplicate subjects in *siblings*, also recurse into
the children. This remove some clutter from deeply nested threads.
Fixes: #2078.
Calculate the thread subject, that is, the subject of the (sub)thread _or_
empty if it's the same as the previous subject.
This is for the UI feature of _not_ showing the subject when it's just
repeating from the previous.
Rewrite the query machinery in c++:
- use an MSet decorator instead of the mu-msg-iter stuff
- use mu-query-decider to mark duplicates/unreadable/related messages
- use mu-query-threader to replace the older container/thread code
Algorithm did not substantially change, but the implementation details
did.