Further simplify use of 'subprocess'.  Two uses of .Popen remain.

* tools/dist/release.py
  (subprocess.check_output): Provide a sufficient replacement for Python 2.6,
    when not available in the library.
  (compare_changes, clean_dist, move_to_dist): Use check_output().  This way,
    non-zero exit code with no stderr would still be detected as an error.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1485573 13f79535-47bb-0310-9956-ffa450edef68