Add svn_diff_mem_string_output_unified3() with cancel support and adjust callers.

* subversion/include/svn_diff.h
  (svn_diff_mem_string_output_unified3): New function that replaces...
  (svn_diff_mem_string_output_unified, svn_diff_mem_string_output_unified2):
    Deprecate.

* subversion/include/private/svn_diff_private.h
  (svn_diff__display_prop_diffs): Add cancel support.

* subversion/libsvn_client/diff.c
  (display_prop_diffs, diff_props_changed): Add cancel support.

* subversion/libsvn_diff/deprecated.c
  (svn_diff_mem_string_output_unified, svn_diff_mem_string_output_unified2): Add
    implementations that are based on svn_diff_mem_string_output_unified3().

* subversion/libsvn_diff/diff_memory.c
  (svn_diff_mem_string_output_unified2): Replace with ...
  (svn_diff_mem_string_output_unified3): New function with cancel support.
  (svn_diff_mem_string_output_unified): Moved to deprecated.c.

* subversion/libsvn_diff/util.c
  (svn_diff__display_prop_diffs): Add cancel support.

* subversion/svnlook/svnlook.c
  (display_prop_diffs): Add cancel support.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618839 13f79535-47bb-0310-9956-ffa450edef68