pgsql: Don't wait for the commit record to be replicated if we wrote no

Don't wait for the commit record to be replicated if we wrote no WAL.

When using synchronous replication, we waited for the commit record to be
replicated, but if we our transaction didn't write any other WAL records,
that's not required because we don't even flush the WAL locally to disk in
that case. This lead to long waits when committing a transaction that only
modified a temporary table. Bug spotted by Thom Brown.

Branch
------
REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/10fcfada235af6186c9474230483ab0fe5c9610d

Modified Files
--------------
src/backend/access/transam/xact.c |    3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)