pgsql: Fix portal management code to support non-default command

Log Message:
-----------
Fix portal management code to support non-default command completion tags for
portals using PORTAL_UTIL_SELECT strategy.  This is currently significant only
for FETCH queries, which are supposed to include a count in the tag.  Seems
it's been broken since 7.4, but nobody noticed before Knut Lehre.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/tcop:
        pquery.c (r1.97.2.1 -> r1.97.2.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c.diff?r1=1.97.2.1&r2=1.97.2.2)