pgsql: Allow pg_upgrade to upgrade an old cluster that doesn't have a

Allow pg_upgrade to upgrade an old cluster that doesn't have a
'postgres' database.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a50d860ae1dfca56148dd41692b963bb859bf1d6

Modified Files
--------------
contrib/pg_upgrade/check.c | 37 +------------------------------------
contrib/pg_upgrade/function.c | 3 +--
contrib/pg_upgrade/relfilenode.c | 22 +++++++++++++++-------
3 files changed, 17 insertions(+), 45 deletions(-)