status and diff: reconsider output and command

[skshetry]

This issue is mostly regarding the UX/UI of those commands, cleaning up its outputs (which is quite messy) and apply the appropriate cli guidelines.
But while we are at it, we should also reconsider these commands and their use cases.

[shcheklein]

Related here iterative/vscode-dvc#772 and it's a very painful and becomes higher and higher priority there. We should collaborate on this from the performance and from the semantics perspective. cc @mattseddon