Reset pg_stat_activity.xact_start during PREPARE TRANSACTION.

Once we've completed a PREPARE, our session is not running a transaction,
so its entry in pg_stat_activity should show xact_start as null, rather
than leaving the value as the start time of the now-prepared transaction.

I think possibly this oversight was triggered by faulty extrapolation
from the adjacent comment that says PrepareTransaction should not call
AtEOXact_PgStat, so tweak the wording of that comment.

Noted by Andres Freund while considering bug #10123 from Maxim Boguk,
although this error doesn't seem to explain that report.

Back-patch to all active branches.

src/backend/access/transam/xact.c

@@ -1963,8 +1963,9 @@ PrepareTransaction(void)
 	AtEOXact_Files();
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(true);
-	/* don't call AtEOXact_PgStat here */
+	/* don't call AtEOXact_PgStat here; we fixed pgstat state above */
 	AtEOXact_Snapshot(true);
+	pgstat_report_xact_timestamp(0);
 
 	CurrentResourceOwner = NULL;
 	ResourceOwnerDelete(TopTransactionResourceOwner);