pgsql: Pg_upgrade C comment addition.

Pg_upgrade C comment addition.

Document why we do the missing new database check during the check
phase.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0262251c337ca066d1b1698684784254849b5676

Modified Files
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contrib/pg_upgrade/check.c | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)