work around noisy 'conflicting chdir during another chdir block' warnings

If the popen method is used with the :chdir option within a Dir.chdir
block on ruby 1.9 (fex inside a in_build_dir block), a warning is issued that is needless distraction here
since we're about to exec anyway.

See the implementation of Dir.chdir
http://www.ruby-doc.org/core-1.9.3/Dir.html#method-c-chdir
Use PathHelpers.cd to wrap the chdir logic and avoid two exec calls.
Merge branch 'chdir-warning'