Running Sup locally
-------------------
Invoke it like this:

ruby -I lib -w bin/sup

Coding standards
----------------

- Don't wrap code unless it really benefits from it. The days of
  80-column displays are long over. But do wrap comments and other
  text at whatever Emacs meta-Q does.
- Use as few parentheses as possible.
- Use {} for one-liner blocks and do/end for multi-line blocks.

How messages are updated in the index
-------------------------------------

Ferret doesn't have any concept of updating; to change message state
it must be deleted then re-added to the index.

Thus there are a couple situations where we'll have a message to be
"added", but it already exists in the index, and we need to decide
which parts of which version to keep:

1. The user has changed the state of the message, e.g. read it or
   added a user label. In this case we want to use the state of the
   version in memory, but keep everything else on disk.

   This is the behavior of Index#update_message

2. We've received a new copy of the message. Crucially, this can
   happen for two different reasons:

   a. The message was sent to a mailing list to which the user is
      subscribed, and we're now getting that message back, possibly
      with altered content (subject mangling, signature adding, etc.)

   b. The user has moved the message between sources. E.g. if the
      primary inbox has a quota, and other sources are on local,
      quota-less disk, the user may regularly move messages from the
      inbox to the sources on disk.

   In both of these cases, the solution is to keep the state from the
   index, but use the new message contents.

   This is the behavior of Index#update_or_add_message, which can be
   also be called for new message.