Revision 877593

Stop merge of difference between two different repositories from
occasionally "working".

Previously, given URL_X and URL_Y which point to two completely different
repositories, the merge,

  svn merge URL_X@M URL_Y@N TARGET_WC

might actually succeed and merge URL_X@M:URL_X@N to TARGET_WC.
                                     ^       ^

* subversion/libsvn_client/client.h
  (svn_client__get_youngest_common_ancestor): Note assumption that path/urls
  being examined are assumed to point to the same repos.  This function can
  find a "common" ancestor between two *different* repositories which have
  similar path structures.

* subversion/libsvn_client/merge.c
  (svn_client_merge3): Raise an error immediately when trying to merge the
  difference between different repositories, don't let things proceed
  because svn_client__get_youngest_common_ancestor's limitations might allow
  some strange (and incorrect) results.