add variable to ignore specific projects

[dradetsky]

I want to add a git repo to my home directory in order to track dotfiles (I'm aware there are other approaches that don't involve creating ~/.git/, but for reasons beyond the scope of this PR, I want to do it this way). When I do this, in doom-emacs doom doctor complains

    ! Your $HOME is recognized as a project root
      Emacs will assume $HOME is the root of any project living under $HOME. If this
      isn't desired, you will need to remove ".git" from
      `projectile-project-root-files-bottom-up' (a variable), e.g.
      
        (after! projectile (setq projectile-project-root-files-bottom-up (remove ".git"
          projectile-project-root-files-bottom-up)))

Maybe I'm misunderstanding something, but it seems like the resolution here isn't something I want to do. I still want projectile to recognize dirs with .git as project roots, I just want it to not consider $HOME to be the universal project root, as threatened by doom doctor. Therefore, I need it to say that one or more named directories are not project roots, regardless of what the root-determination machinery would say otherwise.

So I add a variable to hold a list of such directories, and consult it inside projectile-locate-dominating-file.

NOTE: the PR is intentionally missing some required cleanup (like comments) in case there is something fundamentally wrong with my whole approach (I suck at elisp). That's why it's marked as a draft. AFAIK, the code itself does what it should, but I could be wrong.

---

Before submitting a PR make sure the following things have been done (and denote this
by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

Thanks!

[bbatsov]

The approach seems sound, although I have some small concern about how this will affect performance, as it's an extra check in a function that's used quite often. On the other hand - by default this variable will be nil, so likely the overhead then will be close to none.

One alternative approach to consider would be to use some variable marker set via .dir-locals.el, but I assume some people might not like this, as it will require them to have .dir-local.el in any folder they'd like to ignore.

[dradetsky]

@bbatsov if you're concerned about the performance (I really have no idea whether you're right to be concerned or not), we could check whether projectile-non-project-roots is non-nil before we even bother to expand file & create the closure that we'd map over projectile-non-project-roots. That way, instead of some extra misc elisp stuff per call, it's just an extra compare-to-nil per call (at least for those who aren't using this variable), which I'd assume wouldn't matter (although I haven't benchmarked obviously).

I didn't do this to begin with because I was concerned about the readability of the code, so I tried to avoid getting to clever. But if you think it's worth it here I can do it.

[bbatsov]

I don't think an explicit nil check will be much of a difference - iterating over an empty/nil list is more or less the same. Just pursue your original approach.