Blame callback: remove start/end parameters

The svn_client_blame_receiver4_t parameters "start_revnum" and "end_revnum" do not really belong here because they are not per-line data. They are the "resolved" versions of the input revnums to svn_client_blame6(). I introduced them in r955895, applying a patch by Johan. I propose to move them to svn_client_blame6() output parameters.

(svn_client_blame6() and svn_client_blame_receiver4_t APIs are currently being revved so this is a good time to make such a change.)

-- 
- Julian