On the log-message-templates branch, add an additional API function.

* subversion/include/svn_client.h
  (svn_client_get_log_message_templates_for_commit_items): New. This
   function accepts a list of commit items, instead of a list of paths
   like svn_client_get_log_message_templates() does.

* subversion/libsvn_client/commit_util.c
  (svn_client_get_log_message_templates_for_commit_items): Implement based
   on code from svn_client__get_log_msg().
  (svn_client__get_log_msg): Call svn_client_get_log_message_templates_for_commit_items().


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-message-templates@1623408 13f79535-47bb-0310-9956-ffa450edef68