Add an --incremental option to mergeinfo for use with --log

* subversion/svn/mergeinfo-cmd.c
  (mergeinfo_log): Add incremental argument and don't print log separator
    when it is true.
  (svn_cl__mergeinfo): Reject --incremental option when not used with --log and
    pass state of option to mergeinfo_log().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548262 13f79535-47bb-0310-9956-ffa450edef68