Seperate variable declaration from assigment.

* subversion/libsvn_client/diff.c
  (set_up_diff_cmd_and_options): Seperate variable declaration from assigment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1484260 13f79535-47bb-0310-9956-ffa450edef68