diff: Stats reported by dvc not correct

Description

When tracking a directory using DVC, the stats reported by dvc diff are not correct. For example, in a small test repository, the directory test is being tracked by DVC. When running dvc diff between two commits, the following is reported:

$ dvc diff f722af7 91cdc34

dvc diff from f722af74ec372073d04847a3c77ff7e7a154a21b to 91cdc345625ded08cb50f5616f41cee73b198bb6

diff for 'test'
-test with md5 cd5a9abdb72acc541fecb818c8280d5c.dir
+test with md5 3d681d1fe5b39802f663c9a82600d219.dir

0 files not changed, 0 files modified, 0 files added, 0 files deleted, size was increased by 3 Bytes

It should say that 1 file was modified, but instead only reports the size change correctly.

Information about setup

$ dvc --version
0.35.5

DVC installed via pip.

$ python3 --version
Python 3.6.7

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.2 LTS
Release:	18.04
Codename:	bionic

[GildedHonour]

I fetched those fixes and created a PR.

[shcheklein]

@GildedHonour “0 files not changed”. Yes, I think this is the only difference. We need to fix that. Feel free to fix not changed part so it’s less confusing. We will need to update the docs if needed and write a test for this.