docs: explain why extensions cause dedup to exit
docs: explain why extensions cause dedup to exit

We note in our docs for the dedup command that it will refuse
to de-duplicate the working tree if any Git LFS extensions
are configured, as these have likely caused the object
storage files to differ from what the user expects in their
working tree (e.g., by compression).

github/git-lfs,github/git-lfs,github/git-lfs