Fix pg_rewind regression failure after "fast promotion"
Fix pg_rewind regression failure after "fast promotion"

pg_rewind looks at the control file to determine the server's timeline. If
the standby performs a "fast promotion", the timeline ID in the control
file is not updated until the next checkpoint. The startup process requests
a checkpoint immediately after promotion, so this is unlikely to be an
issue in the real world, but the regression suite ran pg_rewind so quickly
after promotion that the checkpoint had not yet completed.

Reported by Stephen Frost

50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,adam8157/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,greenplum-db/gpdb,ovr/postgres-xl,ashwinstar/gpdb,oberstet/postgres-xl,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,50wu/gpdb,lisakowen/gpdb,yazun/postgres-xl,greenplum-db/gpdb,ovr/postgres-xl,zeroae/postgres-xl,50wu/gpdb,oberstet/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,50wu/gpdb,yazun/postgres-xl,xinzweb/gpdb,yazun/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,xinzweb/gpdb,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,yazun/postgres-xl,ashwinstar/gpdb,oberstet/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ovr/postgres-xl,jmcatamney/gpdb,adam8157/gpdb,ovr/postgres-xl,50wu/gpdb,zeroae/postgres-xl