As a result of confusion about whether the "char" type is signed or
unsigned, scans for index searches like "col < 'x'" or "col <= 'x'"
would start at the middle of the index not the left end, thus missing
many or all of the entries they should find. Fortunately, this
is not a symptom of index corruption. It's only the search logic
that is broken, and we can fix it without unpleasant side-effects.
Per report from Jason Kim. This has been wrong since btree_gin's
beginning, so back-patch to all supported branches.
Discussion: https://postgr.es/m/20210810001649.htnltbh7c63re42p@jasonk.me
Tom Lane