Skip files which are going to be overwritten later
Skip files which are going to be overwritten later

This is a workaround for GNU tar bug which does NOT overwrite symlinks
targeted at root-based targets when unpacking stream. More details are
here: https://gist.github.com/artyom/926ec9c49a2077f2820053274f0b1b16

As a side-effect may increase memory consumption depending on number of
files/layers.

Closes #1

artyom/merge-docker-save