Support merge --record-only alternative that applies only svn:mergeinfo diffs.

* subversion/include/svn_client.h
  (svn_client_merge4): New.
  (svn_client_merge3): Deprecated.
  (svn_client_merge_peg4): New.
  (svn_client_merge_peg3): Deprected. 

* subversion/include/svn_wc.h
  (svn_wc_conflict_resolver_func2_t): Update comment to reflect revved API.

* subversion/libsvn_client/deprecated.c
  (svn_client_merge3, svn_client_merge_peg3): Moved here from merge.c.

* subversion/libsvn_client/merge.c
  (merge_cmd_baton_t): Add new member mergeinfo_diff.
  (merge_props_changed): Apply only svn:mergeinfo props changes during
  --mergeinfo-diff merges.
  (merge_file_changed): Don't apply text changes during --mergeinfo-diff
  merges.
  (merge_file_added, merge_file_deleted, merge_dir_added, merge_dir_deleted):
  Dont add or delete paths during --mergeinfo-diff merges.
  (notification_receiver): Don't show add/delete notifications during
  --mergeinfo-diff merges.
  (do_directory_merge): Update comment to reflect revved API.
  (do_merge): Add arg to indicate a --mergeinfo-diff merge and initialize
  the corresponding member in the merge command baton.
  (merge_cousins_and_supplement_mergeinfo): Update calls to do_merge().
  (svn_client_merge3, svn_client_merge_peg3): Moved to deprecated.c.
  (svn_client_merge4, svn_client_merge_peg4): New...and while we are revving
  change the pool arg to a scratch_pool.

* subversion/svn/cl.h
  (svn_cl__opt_state_t): New mergeinfo_diff member.

* subversion/svn/main.c
  (svn_cl__longopt_t): New opt_mergeinfo_diff enum.
  (svn_cl__options): Add --mergeinfo-diff long option.
  (svn_cl__cmd_table): Add --mergeinfo-diff option to merge subcommand.
  (main): Parse --mergeinfo-diff option.

* subversion/svn/merge-cmd.c
  (svn_cl__merge): Replace calls to svn_client_merge_peg3 and svn_client_merge3
  with svn_client_merge_peg4 and svn_client_merge4.