pgsql: Use SnapshotNow instead of SnapshotSelf for reading the catalogs

Log Message:
-----------
Use SnapshotNow instead of SnapshotSelf for reading the catalogs
during flat-file writing. The only difference is that SnapshotSelf
would consider tuples of the 'current command' within the current
transaction as valid, where SnapshotNow wouldn't. We can eliminate
the need for this with one extra CommandCounterIncrement call before
we start reading the catalogs.

Modified Files:
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pgsql/src/backend/utils/init:
flatfiles.c (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/flatfiles.c.diff?r1=1.2&r2=1.3)