notmuch: reconstruct thread on-the-fly upon entire-thread and no vfolder

notmuch: reconstruct thread on-the-fly upon entire-thread and no vfolder

Before this change <entire-thread> worked only if invoked from a virtual
folder.

Now <entire-thread> works also from regular folders, with the following
semantics:

- a notmuch search is done on the fly using as query the message-id of the
  currently selected message

- mutt switches to the query result (a vfolder)

- <entire-thread> is invoked as before, expanding the thread of the original
  message

Closes #1345