skip realpathing for prefetch on windows
skip realpathing for prefetch on windows

Summary:
Relative paths can not be calculated between drives, so we should not realpath
here because we don't realpath elsewhere.

Reviewed By: MichaelCuevas

Differential Revision: D38378132

fbshipit-source-id: a20d5c5273a2d09166792fbb9862f67d8040fdd3

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