Currently, the error reporting for duplicate key value violation for unique
indexes does not contain useful information for users when debugging. This
commit backports two commits from PostgreSQL (shown below) and updates our
multikey tuplesort to use the same. Now the error will display the first
instance of a duplicate key violation.

Authors: Jimmy Yih and Abhijit Subramanya

commit b680ae4b
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Sat Aug 1 19:59:41 2009 +0000

    Improve unique-constraint-violation error messages to include the exact
    values being complained of.

    In passing, also remove the arbitrary length limitation in the similar
    error detail message for foreign key violations.

    Itagaki Takahiro

 Conflicts:
	contrib/citext/expected/citext.out
	contrib/citext/expected/citext_1.out
	src/backend/access/common/indextuple.c
	src/backend/access/index/genam.c
	src/backend/access/nbtree/nbtinsert.c
	src/backend/utils/adt/ri_triggers.c
	src/backend/utils/adt/ruleutils.c
	src/include/access/genam.h
	src/include/access/itup.h
	src/include/utils/builtins.h
	src/test/regress/expected/alter_table.out
	src/test/regress/expected/arrays.out
	src/test/regress/expected/create_index.out
	src/test/regress/expected/plpgsql.out
	src/test/regress/expected/transactions.out
	src/test/regress/expected/uuid.out
	src/test/regress/output/constraints.source
	src/test/regress/output/tablespace.source

commit 527f0ae3
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Sat Aug 1 20:59:17 2009 +0000

    Department of second thoughts: let's show the exact key during unique index
    build failures, too.  Refactor a bit more since that error message isn't
    spelled the same.

 Conflicts:
	src/backend/access/nbtree/nbtinsert.c
	src/backend/utils/sort/tuplesort.c
	src/test/regress/expected/alter_table.out
	src/test/regress/expected/create_index.out