notmuch-mutt: support for duplicate message removal

[Stefano Zacchiroli]

Heya,
  here is a patchset originating from a feature contributed by Kevin
J. McCarthy: duplicate message removal for notmuch-mutt searches.

I've reviewed the main patch and gone through various iterations of it
with Kevin. I consider it suitable for application in its present
form, and I've added a subsequent patch to fix the Debian packaging
accordingly.

Can someone with commit access be so kind of applying this patchset to
the master branch?

Also, if you've further comments on the patch, do not hesitate!