make sure the paths and cwd are handled properly

We were having an issue with piping multiple tarballs in multiple directories which needed to keep their relative paths

cwd/from/folder/mine.tar
cwd/from/sub/folder/theirs.tar

gulp.src('./from/**/*.tar')
  .pipe(untar())
  .pipe(gulp.dest './deploy')

would end up dropping the folder and sub/folders from the above, instead of keeping the folders in the glob.
This patch fixes that issue