pgsql: Throw error for mismatched pg_upgrade clusters

Throw error for mismatched pg_upgrade clusters

If someone removes the 'postgres' database from the old cluster and the
new cluster has a 'postgres' database, the number of databases will not
match.  We actually could upgrade such a setup, but it would violate the
1-to-1 mapping of database counts, so we throw an error instead.

Previously they got an error during the upgrade, and not at the check
stage; PG 9.0.4 does the same.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7228d02989afd3858ce6eb4de845c56f4c0188a6

Modified Files
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contrib/pg_upgrade/check.c       |   46 ++++++++++++++++++++++++++++----------
contrib/pg_upgrade/relfilenode.c |    6 -----
2 files changed, 34 insertions(+), 18 deletions(-)