Improve heuristics for divining a merge source for the 'svn merge -cN' operation per the comments inlined into the svn_client__suggest_merge_sources() implementation (in libsvn_client). Order the list of recommendations by: 1) The most recent existing merge source. 2) The copyfrom source (which will also be listed as a merge source if the copy was made with a 1.5+ client and server). 3) All other merge sources, most recent to least recent. Determining the order of application of merge sources requires a new RA API.