Revision 1875037

Merge the 'decouple-shelving-cli' branch to trunk.

Summary:

Add the shelving v2 implementation from Subversion 1.11, as an alternative
to the shelving v3 implementation from Subversion 1.12.

They have substantially different pros and cons, so it is beneficial for the
user to be able to choose.

Make the shelving CLI version selectable by an environment variable:
  env. var. not set                 => shelving v3 enabled
  SVN_EXPERIMENTAL_COMMANDS=shelf3  => shelving v3 enabled
  SVN_EXPERIMENTAL_COMMANDS=shelf2  => shelving v2 enabled
  SVN_EXPERIMENTAL_COMMANDS=        => no shelving CLI