Reduce string allocations when working with paths.

[tjni]

Issue: #5541

Context

This rewrites in two places the logic checking if one string path is the start of another so that it does not allocate new strings. I verified in a profile run of my build that the large amount of memory that I observed my build using is no longer present. Please see the attached issue for the original context.

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Gradle Core Team Checklist

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[oehme]

This looks good, I just have one stylistic request :)

I'll merge this next week.

[oehme]

This comment is not necessary, performance tests would catch regressions.

[oehme]

Thank you @tjni!