pgsql: Use fuzzy path cost tiebreaking rule in our oldest supported bra

Use fuzzy path cost tiebreaking rule in our oldest supported branches.

We've been doing it that way since 9.2, cf commit 33e99153e93b9acc,
but some recently-added regression test cases are making a few buildfarm
members fail (ie choose the "wrong" plan) in 9.0 and 9.1 due to
platform-specific roundoff differences in cost calculations. To fix,
back-port the patch that made add_path treat cost difference ratios of
less than 1e-10 as equal.

Branch
------
REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3218f8c33612baca0a1f44d4a243c598ddebad9d

Modified Files
--------------
src/backend/optimizer/util/pathnode.c | 50 +++++++++++++++++++--------------
1 file changed, 29 insertions(+), 21 deletions(-)

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