Support 'svn mergeinfo -rSTART:END' requests where START > END.

Nobody is clamoring for this feature, but the API changes may help me with
an an ongoing issue #4329 'automatic merge uses reintegrate type merge if
source is fully synced' fix.

* subversion/include/svn_client.h

  (svn_client_mergeinfo_log2): Update doc string; previously we didn't
   specify what we did in the case where the start revision was younger
   than the end revision.
 
* subversion/libsvn_client/mergeinfo.c

  (logs_for_mergeinfo_rangelist): Add a new boolean argument signaling that
   the caller wants the merged/eligible revisions from youngest to oldest.

  (svn_client_mergeinfo_log2): Tolerate callers who specify a start revision
   younger than the end revision.  Update call  to
   logs_for_mergeinfo_rangelist.

* subversion/tests/cmdline/mergeinfo_tests.py

  (recursive_mergeinfo): Expand to test the case where merged/eligible
   revisions are requested in youngest to oldest order.

* subversion/tests/cmdline/svntest/actions.py

  (run_and_verify_mergeinfo): Don't sort the expected output, the doc string
   makes no such promises and it prevents us from expecting any output other
   than oldest to youngest.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1463237 13f79535-47bb-0310-9956-ffa450edef68