Implement cancellation support in a few more places of the blame processing.

* subversion/libsvn_client/blame.c
  (add_file_blame): Pass cancellation info to svn_diff_output2().
  (update_blame,
   svn_client_blame5): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1582468 13f79535-47bb-0310-9956-ffa450edef68