pgsql: Reset pg_stat_activity.xact_start during PREPARE TRANSACTION.

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.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b7a3e1173991a147a43dcd3f67b98be051153b4b

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