pgsql: Make archive recovery always start a new timeline, rather than

Log Message:
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Make archive recovery always start a new timeline, rather than only when a
recovery stop time was used.  This avoids a corner-case risk of trying to
overwrite an existing archived copy of the last WAL segment, and seems
simpler and cleaner all around than the original definition.  Per example
from Jon Colverson and subsequent analysis by Simon.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/doc/src/sgml:
        backup.sgml (r2.75.2.2 -> r2.75.2.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.75.2.2&r2=2.75.2.3)
    pgsql/src/backend/access/transam:
        xlog.c (r1.222.2.5 -> r1.222.2.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.222.2.5&r2=1.222.2.6)