Support .gitignore as to tell which paths to not consider, or just consider git tracked files if under git

When people upload from datalad datasets, they might have some incidental files already `git ignored` and it would be a surprise for them to upload some `venvs/` etc. Possible logical ways are
- see if there is `.gitignore` where they should have ignored them .
yet to check if could be used generally to query for each path similarly to how `git check-attr` could be used for `.gitattributes`.
- if dandiset is a git repository, consider only paths under git and ignore the rest by default (would need option to force somehow)