pgsql: pg_rewind: Don't error if the two clusters are already on the sa

pg_rewind: Don't error if the two clusters are already on the same timeline

This previously resulted in an error and a nonzero exit status, but
after discussion this should rather be a noop with a zero exit status.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6b34e5563849edc12896bf5754e8fe7b88012697

Modified Files
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src/bin/pg_rewind/pg_rewind.c            |   59 ++++++++++++++++--------------
src/bin/pg_rewind/t/005_same_timeline.pl |   14 +++++++
2 files changed, 46 insertions(+), 27 deletions(-)