5370 infinite loop caused by symlink

Fixes #5370

When changing from 0.9.1 to 0.10.0 one of my `docker build .` started to hang forever.
It caused by symlinks in the images. Normally symlinks are created as either starting with '.' or '/' character:
```
  ln -s /path/existing /path/new-name
#or
  cd /path && ln -s ./existing new-name
```
but one can create it with realtive file/directory name like:
```
  cd /path && ln -s existing new-name
```
this drives FollowSymlinkInScope into infinite loop

## Fix

I just completed the [The Tour of Go](http://tour.golang.org/) and felt competent enough to fix this issue,
so be warned this my first day with go ;)