pgsql: Prevent the unnecessary creation of .ready file for the timeline

Prevent the unnecessary creation of .ready file for the timeline history file.

Previously .ready file was created for the timeline history file at the end
of an archive recovery even when WAL archiving was not enabled.
This creation is unnecessary and causes .ready file to remain infinitely.

This commit changes an archive recovery so that it creates .ready file for
the timeline history file only when WAL archiving is enabled.

Backpatch to all supported versions.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/38eb5d9e86af67953bad91dd5b9d993b076b37bf

Modified Files
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src/backend/access/transam/xlog.c |    7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)