pgsql: Fix UNION/INTERSECT/EXCEPT so that when two inputs being merged

Log Message:
-----------
Fix UNION/INTERSECT/EXCEPT so that when two inputs being merged have
same data type and same typmod, we show that typmod as the output
typmod, rather than generic -1.  This responds to several complaints
over the past few years about UNIONs unexpectedly dropping length or
precision info.

Modified Files:
--------------
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.345 -> r1.346)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.345&r2=1.346)
        equalfuncs.c (r1.279 -> r1.280)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.279&r2=1.280)
        outfuncs.c (r1.280 -> r1.281)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c.diff?r1=1.280&r2=1.281)
        readfuncs.c (r1.193 -> r1.194)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/readfuncs.c.diff?r1=1.193&r2=1.194)
    pgsql/src/backend/optimizer/path:
        allpaths.c (r1.150 -> r1.151)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c.diff?r1=1.150&r2=1.151)
    pgsql/src/backend/optimizer/prep:
        prepjointree.c (r1.39 -> r1.40)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepjointree.c.diff?r1=1.39&r2=1.40)
        prepunion.c (r1.132 -> r1.133)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c.diff?r1=1.132&r2=1.133)
    pgsql/src/backend/optimizer/util:
        tlist.c (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/tlist.c.diff?r1=1.72&r2=1.73)
    pgsql/src/backend/parser:
        analyze.c (r1.343 -> r1.344)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c.diff?r1=1.343&r2=1.344)
    pgsql/src/include/catalog:
        catversion.h (r1.347 -> r1.348)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.347&r2=1.348)
    pgsql/src/include/nodes:
        parsenodes.h (r1.320 -> r1.321)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.320&r2=1.321)