Factor out some common code and fix a compiler warning.

clang complains about the old code since we potentially pass an extra
argument to svn_stream_printf() that the format doesn't use when the
length isn't 1.

* subversion/libsvn_diff/diff_file.c
  (output_conflict_with_context_marker): New.
  (output_conflict_with_context): Replace 3 instances of the same code with
    calls to output_conflict_with_context_marker().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570666 13f79535-47bb-0310-9956-ffa450edef68