Fixing image lfs bug and also displaying text lfs
This commit, introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23812, fixes a problem creating a displaying image diff notes when the image is stored in LFS. The main problem was that `Gitlab::Diff::File` was returning an invalid valid in `text?` for this kind of files. It also fixes a rendering problem with other LFS files, like text ones. They LFS pointer shouldn't be shown when LFS is enabled for the project, but they were.
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