notmuch-mutt: switch to using the new --duplicate flag

[Kevin McCarthy]

This patch series changes notmuch-mutt to use the new built-in
--duplicate flag for removing duplicates by message-id.

This should speed up duplicate removal, since we no longer need to scan
and compute sha sums.  However, it does now allow search results to be
hidden in the event of accidental or malicious message-id re-use.

The duplicate removal is optional, but is currently enabled by default
in the distributed /etc/Muttrc.d/notmuch-mutt.rc file.

Please don't hesitate to comment if this change is too controversial!

Thank you,

-Kevin