In SvnPubSub, extract wrapper for subprocess.check_output into a common utility
module. And make problems in the server hooks more visible by exiting with an
error if the hook fails to run.

[in tools/server-side/svnpubsub]
* svnpubsub/util.py: New module.
  (check_output): Wrapper for subprocess.check_output to make it available
   on versions of Python older than 2.7.

* svnwcsub.py (check_output): Removed. Imported from svnpubsub.util instead.
  (svn_info): Updated call to check_output.

* commit-hook.py: Do 'import svnpubsub.util' and do not 'import subprocess'.
  (svnlook): Renamed from svncmd; calls svnpubsub.util.check_output and
   expects the command to be a list, not a string. All callers updated.
  (svnlook_uuid): Renamed from svncmd_uuid. All callers updated.
  (svnlook_info): Renamed from svncmd_info. All callers updated.
  (svnlook_changed): Renamed from svncmd_changed. All callers updated.
  At top level: Exit with an error code if the number of arguments is wrong.

* revprop-change-hook.py: Same changes as in commit-hook.py, except that the
   renamed functions are: svnlook (from svncmd), svnlook_uuid (from svncmd_uuid)
   and svnlook_revprop (from svncmd_revprop).


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486500 13f79535-47bb-0310-9956-ffa450edef68