data status: show untracked files in normal mode

From https://github.com/iterative/dvc.org/pull/3812#discussion_r931002653:

Is it a performance optimization?

It no longer is, or will no longer be of performance concern. We didn’t have --untracked-files=normal support in dulwich so we had to use --untracked-files=all, so it’d be noisy and slow if you have too many untracked files.

But thanks to @dtrifiro’s great work in pygit2, the git.status() is about 30x faster. Plus, we now have --untracked-files=normal support in pygit2, which will be much faster even for a very large repository. So there’s no performance issue now if we decide to revisit. See iterative/scmrepo#118.

(I have been testing a repo with 150,000 untracked files from a dataset, and --untracked-files=normal takes ~20ms now)

[skshetry]

Maybe we can even get rid of the flag, enable --untracked-files=normal by default and change to --untracked-files=all when --granular is used.

[skshetry]

@dberenbaum, we have the support for this in upstream now.