Projectile has `projectile-git-command` and
`projectile-git-submodule-command`. By default, these use `git ls-files`
and `git submodule foreach 'echo $path'`, respectively. It uses them
together to index a full list of project files.
Since we've changed `projectile-git-command` to use `fd` or `ripgrep`,
which indexes submodules just fine (unlike git ls-files), you get
duplicates when projectile does a second pass with
`projectile-git-submodule-command`, so we unset it completely.
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