pgsql: Clean up MergeAttributesIntoExisting()

Clean up MergeAttributesIntoExisting()

Make variable naming clearer and more consistent.  Move some variables
to smaller scope.  Remove some unnecessary intermediate variables.
Try to save some vertical space.

Apply analogous changes to nearby MergeConstraintsIntoExisting() and
RemoveInheritance() for consistency.

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eddad679d2ab6d7369767d1c2f01fdbe76afbce3

Modified Files
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src/backend/commands/tablecmds.c | 123 +++++++++++++++------------------------
1 file changed, 48 insertions(+), 75 deletions(-)