pgsql: Further marginal speed hacking: in MemoryContextReset, don't call

Log Message:
-----------
Further marginal speed hacking: in MemoryContextReset, don't call
MemoryContextResetChildren unless necessary.

Modified Files:
--------------
pgsql/src/backend/utils/mmgr:
mcxt.c (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/mcxt.c.diff?r1=1.54&r2=1.55)