pgsql: MergeAttributes() and related variable renaming

MergeAttributes() and related variable renaming

Mainly, rename "schema" to "columns" and related changes.  The
previous naming has long been confusing.

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/b0ae29512c2172f88696fe0963dd2a3304dc4972

Modified Files
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src/backend/access/common/tupdesc.c |  10 ++--
src/backend/commands/tablecmds.c    | 109 +++++++++++++++++-------------------
src/include/access/tupdesc.h        |   4 +-
3 files changed, 59 insertions(+), 64 deletions(-)