chroot: allow Symlinks cross boundary

[mcuadros]

This PR allows the creation of links where the target is outside of the chroot. The downside is that the file can be read it, not being a strict chroot anymore.

Fixes src-d/go-git#445

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #38      +/-   ##
=========================================
+ Coverage   86.47%   86.9%   +0.42%     
=========================================
  Files           8       8              
  Lines         658     649       -9     
=========================================
- Hits          569     564       -5     
+ Misses         56      54       -2     
+ Partials       33      31       -2

Impacted Files	Coverage Δ
helper/chroot/chroot.go	89.91% <ø> (+2.41%)	⬆️

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