Log Message:
notmuch: Update to 0.31

Changelog:
What's new in notmuch 0.31
=========================

Emacs
-----

Notmuch now supports Emacs 27.1. You may need to set
`mml-secure-openpgp-sign-with-sender` and/or
`mml-secure-smime-sign-with-sender` to continue signing messages.

The minimum supported major version of GNU Emacs is now 25.1.

Add support for moving between threads after notmuch-tree-from-search-thread.

New `notmuch-unthreaded` mode (added in Notmuch 0.30)

  Unthreaded view is a mode where each matching message is shown on a
  separate line.

  The main key entries to unthreaded view are

  'u' enter a query to view in unthreaded mode (works in hello,
      search, show and tree mode)

  'U' view the current query in unthreaded mode (works from search,
      show and tree)

  Saved searches can also specify that they should open in unthreaded
  view.

  Currently it is not possible to specify the sort order: it will
  always be newest first.

Notmuch-Mutt
------------

The shell pipeline executed by notmuch-mutt, which symlinked matched
files to a maildir for mutt to access is replaced with internal perl
processing. This search operation is now more portable, and somewhat
faster.

Library
-------

Improve exception handling in the library. This should
largely eliminate terminations inside the library due to uncaught
exceptions or internal errors.  No doubt there are a few uncovered
code paths still; please report them as bugs.

Add `notmuch_message_get_flag_st` and
`notmuch_message_has_maildir_flag_st`, and deprecate the existing
non-status providing versions.

Move memory de-allocation from `notmuch_database_close` to
`notmuch_database_destroy`.

Handle relative filenames in `notmuch_database_index_file`, as
promised in the documentation.

Python Bindings
---------------

Documentation for the python bindings is merged into the main
sphinx-doc documentation tree. The merged documentation can be built
with e.g. `make sphinx-html`

Dependencies
------------

We now support building notmuch against Xapian 1.5 (the current
development version).

Test Suite
----------

Test suite fixes for compatibility with Emacs 27.1.

Build System
------------

Man pages are now compressed reproducibly.