On the 'move-tracking-2' branch: Replace a pair of functions with one that
is a bit better.

* subversion/svnmover/svnmover.c
  (family_find_element_by_path,
   family_get_branch_by_path): Remove.
  (branch_find_nested_branch_element_by_path): New.
  (repos_get_branch_by_path): Update to use the new function. Add an
    optional element id output parameter, as it will likely be useful in future.
    Update callers.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1627512 13f79535-47bb-0310-9956-ffa450edef68