pull/fetch: show how many files were downloaded

[skshetry]

#3401 is merged. This introduced showing changes during checkout. pull is a fetch + checkout, so, the output is a bit mixed. It'd be good to get statistics of what was downloaded during fetch or even better, what files were downloaded.

[natikgadzhi]

Yes please!

I'm a new DVC user, working on a hobby app with a friend. I've pushed a few files, and then when the friend ran dvc pull, he just got a console output of 1 added.

1 added is not exactly friendly or clear on what happened (which is, one data object containing a bunch of files was added to the local dvc env, I guess).