Audit all libnotmuch entry points to ensure that all Xapian calls are
wrapped in a try/catch block.
+Fix the "count" functionality to be exact as Olly explained in IRC:
+
+ ojwb> cworth: if you set the check_at_least parameter to the
+ database size, get_matches_estimated() will be exact
+
Search syntax
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Implement support for "tag:*" to expand to all tags.