Add '--transaction' support to 'svnadmin setrevprop' to provide a
way to set txn-props that doesn't require using the bindings.

* subversion/svnadmin/svnadmin.c
  (cmd_table): Add 't' to setrevprop.
  (set_revprop): Set txn-prop.
  (subcommand_setrevprop): Sanity check the new option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631435 13f79535-47bb-0310-9956-ffa450edef68