Unfortunately the functionality you're requesting is not currently implemented in FishEye. Nowadays, it'll read all text files in order to calculate the LOC.
I would recommend creating a new issue in our development issue tracker for this functionality to ensure our development team is aware of your request:
For more information on how Atlassian implements new features and improvements please see the following document:
My apologies that I don't have a solution for you at this time.
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