Log Message:
notmuch: update to 0.26.2.

Notmuch 0.26.2 (2018-04-28)
===========================

Library Changes
---------------

Work around Xapian bug with `get_mset(0,0, x)`.

  This causes aborts in `_notmuch_query_count_documents` on
  e.g. Fedora 28.  The underlying bug is fixed in Xapian commit
  f92e2a936c1592, and will be fixed in Xapian 1.4.6.

Make thread indexing more robust against reference loops

  Choose a thread root by date in case of reference loops. Fix a
  related abort in `notmuch show`.