Fix a bug and greatly improve merge performance in a common use case.

Quite a while back glasser pointed out that the code to filter
self-referential mergeinfo was a performance hog - see
http://svn.haxx.se/dev/archive-2008-02/0206.shtml.  More recently arfrever
found a merge bug where this filtering code removed valid mergeinfo - see
http://subversion.tigris.org/servlets/ReadMsg?list=dev&&msgNo=142849.
This change implements glasser's suggestion, which also fixes the problem
arfrever noted.

Suggested by: glasser

* subversion/libsvn_client/merge.c
  (split_mergeinfo_on_revision): New.
  (filter_self_referential_mergeinfo): Where possible use
  svn_client__get_history_as_mergeinfo() one time per incoming mergeinfo
  merge source rather than svn_client__repos_locations() for each discrete
  range in each merge source's rangelist.

* subversion/tests/cmdline/merge_tests.py
  (natural_history_filtering): Remove comment about XFail.
  (test_list): Remove XFail from natural_history_filtering.