pgsql-server: Now that xmax and cmin are distinct fields again, we

Log Message:
-----------
Now that xmax and cmin are distinct fields again, we should zero xmax when
creating a new tuple. This is just for debugging sanity, though, since
nothing should be paying any attention to xmax when the HEAP_XMAX_INVALID
bit is set.

Modified Files:
--------------
pgsql-server/src/backend/access/heap:
heapam.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/heap/heapam.c.diff?r1=1.175&r2=1.176)