Diff staged data similar to `git diff --cached`

[YifuTao]

With git diff --cached, one could check the staged data. This is useful before commiting the data. Currently, dvc diff does not support showing staged data in a similar way. It only supports diffing between two git commits, which means you have to git commit first before checking what's actually committed.

It would be useful if dvc could implement something like git diff --cached.

Context: https://discordapp.com/channels/485586884165107732/563406153334128681/1134156428077191198

[anmol-hpe]

@skshetry I am willing to work on this

[skshetry]

I think this is no longer necessary. There is dvc data status command that can show you staged changes.

[anmol-hpe]

Okay, let me find some other area to work. Thanks @skshetry