notmuch-mutt: support for duplicate message removal

[Kevin J. McCarthy]

Jani Nikula wrote:
> I'm guessing you get the duplicates because you have dupes in the mail
> store, and 'notmuch search --output=files' prints all the filenames
> associated with each matching message, rather than any other reason. The
> presented approach will only remove identical files, and will leave
> behind files that are basically the same message, but have differing
> headers, e.g. due to being received through different channels. Is this
> what you want?

This method was something we felt comfortable enabling by default.

Stefano and I discussed adding a (by default off) option to remove
duplicates by message-id, but wanted to get this patch merged first and
then think about it.

> Perhaps an option to 'notmuch search --output=files' to print only one
> filename (even if there are many) per message would be interesting.

This would be useful for the second approach.  If it's easy to do, that
would be great.

-Kevin