pull/fetch/push: add stats

[skshetry]

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• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

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Closes #3485

$ dvc pull
M	bar
A	foo
1 file added, 1 file modified and 2 files fetched

$ dvc push
2 files pushed

$ dvc fetch
1 file fetched

[pared]

@skshetry seems like you need to modify test_data_sync unit command tests. Now we are mocking fetch and push return value, and logger is unable to parse it.

[skshetry]

Thanks @pared, I have adjusted the tests. I have set return_value, now the mock should be properly set.

[efiop]

@skshetry Are you sure we don't need docs for this PR ?