Use abspath if relpath fails (windows bug fix)

If I'm syncing a directory of d:\something and my cwd is c:\something
then os.path.relpath complains that it can't work out a relative path.
When this happens, we just use the abspath instead.
Merge branch 'no-relpath' into develop

* no-relpath:
  Use abspath if relpath fails (windows bug fix)