dvc: adding external file through a symlink to a directory

https://discordapp.com/channels/485586884165107732/485596304961962003/548151166706647064

```
#!/bin/bash

mkdir dvc_example
cd dvc_example

# Create data dir
mkdir data
echo "data" >> data/data_file

mkdir my_repo
cd my_repo
ln -s ../data .

git init
dvc init

dvc add -v data/data_file

```