make mergeinfo diagram column headings l10n friendly

* subversion/svn/mergeinfo-cmd.c
  (mergeinfo_diagram): use one line per column heading and
   vertical lines, leaving room for l10n

Approved by: julianfoad
             brane


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1438400 13f79535-47bb-0310-9956-ffa450edef68