pgsql: Fix problems with certain shells (e.g., FreeBSD, Cygwin) clearing

Log Message:
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Fix problems with certain shells (e.g., FreeBSD, Cygwin) clearing the
exit status in multiline traps.

Modified Files:
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pgsql/src/test/regress:
pg_regress.sh (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/pg_regress.sh.diff?r1=1.50&r2=1.51)