dvcignore/tree benchmark

[pared]

Create benchmark utilizing dvcignore functionality.
We need to see how checking ignores each time we call exist (treeverse/dvc#3876) influences DVC.

I think we should create a benchmark that will consist of 3 tests:

1. Timing traversing when there is no dvcignore's
2. ^^ but with some dvcignores.
3. ^^ but with many dvcignores.
   "some" and "many" exact definitions need to be narrowed down.