`dvc diff` calls a file change "added file" (with negative size)

[jorgeorpinel]

Please provide information about your setup
DVC version 0.35.7 installed with Homebrew on macOS v. 10.14.4

To reproduce

git clone https://github.com/iterative/example-get-started
cd example-get-started
dvc fetch -T
dvc diff -t model.pkl baseline-experiment bigrams-experiment

Problem
The output of the last command above is

dvc diff from bc1722d7eeb4cba9a5c8e401199e995739c474a9 to 8c1169d1819c5cf0a4e2aa7e7d8c43854563b251

diff for 'model.pkl'
-model.pkl with md5 a66489653d1b6a8ba989799367b32c43
+model.pkl with md5 3863d0e317dee0a55c4e59d2ec0eef33

added file with size -202464 Bytes

Why "added file" ? The model.pkl file changed.

[jorgeorpinel]

May be related to #1874

[jorgeorpinel]

This is the first issue I ever opened, some time ago @shcheklein. What labels should it have? Thanks 😃