pgsql: Assert that partition inherits from only one parent in MergeAttr

Assert that partition inherits from only one parent in MergeAttributes()

A partition inherits only from one partitioned table and thus inherits
a column definition only once.  Assert the same in MergeAttributes()
and simplify a condition accordingly.

Similar definition exists about line 3068 in the same function.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAExHW5uOykuTC+C6R1yDSp=o8Q83jr8xJdZxgPkxfZ1Ue5RRGg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d22d98c341713a260c1d821a62e83c19e27defa0

Modified Files
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src/backend/commands/tablecmds.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)