pgsql: Prevent msys2 conversion of "cmd /c" switch to a file path

Prevent msys2 conversion of "cmd /c" switch to a file path

Modern versions of msys2 have changed the treatment of "cmd /c" so that
the runtime will try to convert the switch to a native file path. This
patch adds a setting to inhibit that behaviour.

Discussion: https://postgr.es/m/3227042f-cfcc-745a-57dd-fb8c471f8ddf@2ndQuadrant.com

Backpatch to all live branches.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/801b2618ef385eb877633eeab925b82661387ced

Modified Files
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src/bin/pg_upgrade/test.sh | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)