Implement 'svnadmin info'.

* subversion/svnadmin/svnadmin.c
  (subcommand_info): New subcommand.
  (cmd_table): Hook it to the vtable.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1471717 13f79535-47bb-0310-9956-ffa450edef68