Is your feature request related to a problem? Please describe.

Hi,

I'm switching to jj from git as my primary way of interacting with upstream git repos. (It's really good, i can't praise it enough). However, this causes some issues when I don't use a co-located git repo.

Nix decides that it's not a git repo (fair), and then proceeds to copy everything into the store, including otherwise .gitignore or .ignored files (unfortunate).

Besides this being bloaty/wasteful, this includes things like postgres state files, which are not able to be copied to the store.

Describe the solution you'd like

If a .ignore is present (maybe symlinked to .gitignore), nix will use that as an exclude filter when copying to the store.

Describe alternatives you've considered

Not much, I assume there's .hgignore support, or whatnot, this feels like an easy/safe win at first glance.

Additional context
n/a

Priorities

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