Option to diff content of git-lfs tracked files that have 'diff=lfstext' in '.gitattributes'
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### Proposal
`git-lfs` mentions in its [FAQ](https://github.com/git-lfs/git-lfs/wiki/FAQ) a way to set up a flow where one can text-based diff LFS stored/tracked files. I think it would be great if GitLab could support diffing files that are tracked with `diff=lfstext` in the `.gitattributes` file. `lfstext` is just a convention mentioned in the FAQ, and a developer can define a custom global converter with any name. However, I think just tracking this convention and supporting text-based diffing of certain LFS files would be very helpful.
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