Add svn_diff_mem_string_output_merge3() with cancel support.

* subversion/include/svn_diff.h
  (svn_diff_mem_string_output_merge3): New function.
  (svn_diff_mem_string_output_merge2): Deprecate.

* subversion/libsvn_diff/diff_memory.c
  (svn_diff_mem_string_output_merge2): Convert into ...
  (svn_diff_mem_string_output_merge3): New function with cancel support.

* subversion/libsvn_diff/deprecated.c
  (svn_diff_mem_string_output_merge2): New implementation that calls
    svn_diff_mem_string_output_merge3().

* subversion/libsvn_wc/conflicts.c
  (generate_propconflict): Add cancel support and use *_merge3().
  (svn_wc__conflict_invoke_resolver): Update to pass cancel args to
    generate_propconflict.

* subversion/libsvn_wc/props.c
  (prop_conflict_from_skel, append_prop_conflict, svn_wc__create_prejfile):
    Add cancel support.

* subversion/libsvn_wc/props.h
  (svn_wc__create_prejfile): Add cancel support.

* subversion/libsvn_wc/workqueue.c
  (run_prej_install): Pass cancel func/baton to svn_wc__create_prejfile.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618599 13f79535-47bb-0310-9956-ffa450edef68