On the invoke-diff-cmd branch: Repair erroneous initialization.

* subversion/svn/io.c

  (svn_io_run_external_diff): Change pcalloc to palloc call.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1485007 13f79535-47bb-0310-9956-ffa450edef68