Fix issue #4301, 'svn mergeinfo' won't work on moved target.

* subversion/libsvn_client/merge.c
   (svn_client_get_merging_summary): Treat svn_opt_revision_base as a local
    target revision, rather than as a repository target revision. The target
    path might only exist in the working copy, e.g. in case it was locally
    copied or moved. When this happened 'svn mergeinfo' ran into a dubious
    error because it tried to query history of a non-existent repository path.
    So for the purposes of this function, deriving the in-repos location for
    svn_opt_revision_base from the working copy path is a better idea.

* subversion/svn/mergeinfo-cmd.c
  (mergeinfo_summary): Same change as for svn_client_get_merging_summary()
   since the same logic is duplicated here.

* subversion/tests/cmdline/mergeinfo_tests.py
  (mergeinfo_local_move, test_list): New regression test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545986 13f79535-47bb-0310-9956-ffa450edef68