pgsql: Avoid unnecessary palloc overhead in _bt_first().

Log Message:
-----------
Avoid unnecessary palloc overhead in _bt_first().  The temporary
scankeys arrays that it needs can never have more than INDEX_MAX_KEYS
entries, so it's reasonable to just allocate them as fixed-size local
arrays, and save the cost of palloc/pfree.  Not a huge savings, but
a cycle saved is a cycle earned ...

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtsearch.c (r1.92 -> r1.93)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.92&r2=1.93)