Optimize EngineFileUtilities.GetFileList #6227
Tweaked the code under
Existing unit tests for correctness, ETL traces for performance.
This change together with #6151 is showing about 30% less time spent in this particular function when building a Hello World .NET Core project with the Framework version of MSBuild. It's an OK win for project evaluation perf but translates to less than 1 ms of total command line build time.
This PR is small but still recommended to be reviewed commit by commit.