cli/{show,reply}: use repaired form of "Mixed Up" mangled messages

When showing or replying to a message that has been mangled in transit
by an MTA in the "Mixed up" way, notmuch should instead use the
repaired form of the message.

Tracking the repaired GMimeObject for the lifetime of the mime_node so
that it is cleaned up properly is probably the trickiest part of this
patch, but the choices here are based on the idea that the
mime_node_context is the memory manager for the whole mime_node tree
in the first place, so new GMimeObject tree created on-the-fly during
message parsing should be disposed of in the same place.

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