BUG #15896: pg_upgrade from 10-or-earlier: TRAP: FailedAssertion(»!(metad->btm_version >= 3)«

The following bug has been logged on the website:

Bug reference:      15896
Logged by:          Christoph Berg
Email address:      myon@debian.org
PostgreSQL version: 12beta2
Operating system:   Debian
Description:        

When pg_upgrading from 10-or-earlier to 12beta2 or 13devel, an assertion is
raised. (Spotted by Debian's postgresql-common upgrade tests. Previously
missed because we were only testing version+1 upgrades.)

TRAP: FailedAssertion(»!(metad->btm_version >= 3)«, Datei:
»/build/postgresql-12-3URvLF/postgresql-12-12~beta2/build/../src/backend/access/nbtree/nbtpage.c«,
Zeile: 665)

Reproducer:
10.9:
CREATE TABLE phone (name varchar(255) PRIMARY KEY, tel int NOT NULL);
INSERT INTO phone VALUES ('Alice', 2);
CREATE USER foo;
GRANT INSERT ON phone TO foo;
INSERT INTO phone VALUES ('Bob', 1);

pg_upgradecluster -m upgrade 10 foo
(runs /usr/lib/postgresql/12/bin/initdb -D /var/lib/postgresql/12/foo
--auth-local peer --auth-host md5 --encoding UTF8 --lc-collate de_DE.utf8
--lc-ctype de_DE.utf8
/usr/lib/postgresql/12/bin/pg_upgrade -b /usr/lib/postgresql/10/bin -B
/usr/lib/postgresql/12/bin -p 5434 -P 5435 -d /etc/postgresql/10/foo -D
/etc/postgresql/12/foo)

12beta2:
SELECT * FROM phone ORDER BY name;
TRAP: FailedAssertion(»!(metad->btm_version >= 3)«, Datei:
»/build/postgresql-12-3URvLF/postgresql-12-12~beta2/build/../src/backend/access/nbtree/nbtpage.c«,
Zeile: 665)
2019-07-05 11:34:35.040 CEST [14325] LOG:  Serverprozess (PID 14376) wurde
von Signal 6 beendet: Abgebrochen
2019-07-05 11:34:35.040 CEST [14325] DETAIL:  Der fehlgeschlagene Prozess
führte aus: SELECT * FROM phone ORDER BY name;

Combinations tested:
bad: 10->12 9.6->12 9.6->13
good: 10->11 11->12 12->13