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