On the 'move-tracking-2' branch: Split the element-based branching code out
into separate files.

* subversion/include/private/svn_editor3.h
  Make more APIs (semi-)public.

* subversion/libsvn_delta/branching.c
  New file, implementing element-based branch/move tracking.

* subversion/libsvn_delta/compat3b.c
  New file, implementing the element-based Ev3-to-Ev1 converter.

* subversion/libsvn_delta/compat3.c
  Remove the element-based branch/move tracking code and Ev3-to-Ev1 
  converter, leaving only the sequential path-based Ev3<>Ev1 converters.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1639592 13f79535-47bb-0310-9956-ffa450edef68