Improve labelling of the file diff in interactive conflict resolution.  Show
labels like

  --- THEIRS - relpath/to/file.r1234
  +++ MERGED - relpath/to/file

instead of like

  --- /home/julianfoad/tmp/svn/conflicts/wc/f.r1234  Thu Apr 18 15:17:49 2013
  +++ /home/julianfoad/tmp/svn/conflicts/wc/f  Thu Apr 18 15:17:49 2013

This applies only to the 'df' option which shows a 2-way diff for a text
file; the 'dc' 3-way diff option already uses labels such as 'THEIRS' and
'MINE'.

* subversion/svn/conflict-callbacks.c
  (show_diff): Construct labels for the before and after sides of the diff.
    Take a path prefix parameter and use it to display shorter paths.
  (handle_text_conflict): Pass the path prefix to show_diff().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1470238 13f79535-47bb-0310-9956-ffa450edef68