Prior to 9ad19e4454 there was an unhandled Xapian exception when
reindexing after a large number of deletes. This test was used for
bisection, and will subsequently serve as a regression test.
strncmp looks for a prefix that matches, which is very much not what
we want here. This fixes the bug reported by Franz Fellner in
id:1588595993-ner-8.651@TPL520
In id:1588595993-ner-8.651@TPL520 Franz Fellner reported that tags
starting with 'attachment' are removed by 'notmuch reindex'. This is
probably related to the use of STRNCMP_LITERAL in
_notmuch_message_remove_indexed_terms.
