check-ignore: stabilize behavior for subrepos

[efiop]

git chec-ignore will error-out if you try to use a path that is in submodule. Seems like dvc check-ignore should do the same with our subrepos.

See https://github.com/iterative/dvc/pull/6000#discussion_r631438598

[karajan1001]

git check-ignore didn't error out but showed null on my computer. But still different than dvc check-ignore

[efiop]

@karajan1001 is subrepo a git submodule?

[karajan1001]

@karajan1001 is subrepo a git submodule?

Both Git and DVC.

[karajan1001]

OK, I know I didn't commit it.

[karajan1001]

fatal: ../a.dvc: '../a.dvc' is outside repository at '/Users/gao/Code/test/subrepo/subrepo'

Same condition on files outside repo.