While working on diff, resolve a long standing personal issue by making it
possible to cancel unified diff output.

* subversion/include/svn_diff.h
  (svn_diff_output2): New function.
  (svn_diff_output): Deprecate function.
  (svn_diff_file_output_unified4): New function.
  (svn_diff_file_output_unified3): Deprecate function.

* subversion/libsvn_client/diff.c
  (diff_writer_info_t): Add cancel callback.
  (diff_content_changed): Update caller.
  (svn_client_diff6,
   svn_client_diff_peg6): Set cancel in baton.

* subversion/libsvn_diff/deprecated.c
  (svn_diff_file_output_unified3,
   svn_diff_output): New function.

* subversion/libsvn_diff/diff_file.c
  (svn_diff_file_output_unified3): Rename to...
  (svn_diff_file_output_unified4): ... and add cancel support.

* subversion/libsvn_diff/util.c
  (svn_diff_output): Rename to...
  (svn_diff_output2): ... and add cancel support.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570149 13f79535-47bb-0310-9956-ffa450edef68