Make test-restrospective.sh test script compatible with OSX
Replace heavyweight use of python with a simple 'pwd' call
Use custom fullpath() fn to avoid confusion with realpath vs alternative

* Define and use simple `pwd`-based fullpath() function
* Remove misleading comment

Confirmed to work on OSX.
Merge branch 'osx_test_compat_fixes' of github:jmurty/git-fat (PR #35)

* 'osx_test_compat_fixes' of github:jmurty/git-fat:
  Use custom fullpath() fn to avoid confusion with realpath vs alternative
  Replace heavyweight use of python with a simple 'pwd' call
  Make test-restrospective.sh test script compatible with OSX