update fid column in db for changes in comment_upload

comment_upload's 5.x -> 6.x conversion resulted in a renumbering of the fid column in {comment_upload}, due to the new file API, so pift_data.fid is incorrect for all files attached to issues until this is fixed. the old fid data should be available in comment_upload.legacy_fid -- i'm waiting on the author of comment_upload to push this change to the module, then i'll write an update function for PIFT.