index: repair "Mixed Up" messages before indexing.

When encountering a message that has been mangled in the "mixed up"
way by an intermediate MTA, notmuch should instead repair it and index
the repaired form.

When it does this, it also associates the index.repaired=mixedup
property with the message.  If a problem is found with this repair
process, or an improved repair process is proposed later, this should
make it easy for people to reindex the relevant message.  The property
will also hopefully make it easier to diagnose this particular problem
in the future.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>