ignore-check for DVC?

[karajan1001]

In #4249 #4110 #4103 etc. We need to check if some files were ignored or not.
Current solutions are something like,

python -c "from dvc.repo import Repo;print(list(Repo().tree.walk_files('data')))"
# or 
dvc pull -v subdata1.dvc | grep "xml"
# etc. 

These unofficial usages are neither elegant nor detailed. While a git ignore-check can tell you which pattern matches and finally influences your file.

This is helpful when we are writing some complex .dvcignore patterns or debugging.

[skshetry]

@karajan1001, is this the duplicate of #3736?

[karajan1001]

@skshetry Haha, I didn't know there already has been one.