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.

This is a back-patch of commit 6b34e5563849edc12896bf5754e8fe7b88012697,
plus two changes from commit e50cda78404d6400b1326a996a4fabb144871151
that teach pg_rewind to allow the initial control file states to be
DB_SHUTDOWNED_IN_RECOVERY as well as DB_SHUTDOWNED. That's necessary
to get the additional regression test case to pass, and the old behavior
seems like rather a foot-gun anyway.

Peter Eisentraut and Tom Lane

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/4b021aa6103f49bece51d58fd5e307d4d03380bc

Modified Files
--------------
src/bin/pg_rewind/pg_rewind.c | 68 +++++++++++++++++-------------
src/bin/pg_rewind/t/005_same_timeline.pl | 14 ++++++
2 files changed, 52 insertions(+), 30 deletions(-)