ignore: remove dulwich in favour of pathspec

[pared]

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related to #1876

Edit:
When (if) this change is approved, we should create issue to optimize pathspec usage. Now, during dvc_walk we check files one by one, while pathspec allows to check set/list of paths against spec.

Edit 2:
But first it might be better to do some performance testing if its worth our time

[Suor]

Looks like straightforward simplification 👍

The whole ignore thing looks too complicated though, but this PR is certainly not about that.

[pared]

@Suor About ignore complication: lets continue this discussion in #2161 , I think it will be strictly related to this task.

[Suor]

@pared can't see so how #2161 is related to the whole complexity of the ignore implementation. Looks like that does particular feature/bug fix.