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This reintroduces the change from commit v3.10.0-rc1~69^2 (Performance: Improve efficiency of source file lookup in cmMakefile, 2017-08-17) with some corrections. The original was rolled back by commit v3.10.0-rc1~52^2~1 (Revert "Performance: ...", 2017-09-25) due to incompatibilities found. The rollback was followed-up by addition of a test for the offending case, and this revision passes the test.
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