On the invoke-diff-cmd branch: Improve call to apr_pstrcat.

* subversion/libsvn_subr/io.c

  (svn_io_run_external_diff): Improve call to apr_pstrcat.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1489096 13f79535-47bb-0310-9956-ffa450edef68