Rework 'svn mergeinfo' in light of now-mergeinfoless copies.
Rework 'svn mergeinfo' in light of now-mergeinfoless copies.

* subversion/libsvn_client/mergeinfo.c
  (get_mergeinfo): New, abstracted from...
  (svn_client_mergeinfo_get_merged): ...this.
  (svn_client_mergeinfo_get_available): Rework this in light of mergeinfoless
    copies.  While here, teach this thing to examine all the history of
    the sources and potential target, which provides much more reliable
    and useful output.
    


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@868213 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion