A small bit of issue #4351 'improve automatic merge performance' work: Avoid
one to two network roundtrips to get the *same* mergeinfo catalogs while
calculating automatic merges.

* subversion/include/private/svn_client_private.h

  (svn_client__mergeinfo_log): New, wrapper around
   svn_client_mergeinfo_log2(). 

* subversion/libsvn_client/merge.c

  (short_circuit_mergeinfo_log): Wrap svn_client__mergeinfo_log instead of
   svn_client_mergeinfo_log2 (yes, it's a wrapper of a wrapper).  Add a
   in/out parameter for the target's mergeinfo catalog.

  (find_last_merged_location): Update calls to short_circuit_mergeinfo_log().
   On the first call, let the callee get the target's mergeinfo catalog.  On
   the second call, pass the callee the target's mergeinfo catalog, thus
   preventing it from asking the repository the same question twice.

* subversion/libsvn_client/mergeinfo.c

  (svn_client__mergeinfo_log): New, but mostly the old implementation of
   svn_client_mergeinfo_log2().

  (svn_client_mergeinfo_log2): Now just a wrapper around
   svn_client__mergeinfo_log().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1478987 13f79535-47bb-0310-9956-ffa450edef68