Extract a helper function from the file-based unified diff output code.

* subversion/libsvn_diff/diff_file.c
  (output_unified_diff_range): New function.

  (output_unified_flush_hunk): Use helper instead of local loop.

  (output_unified_diff_modified): Use helper instead of loops. Use two
    separate variables to help in document what this code does.
    Don't conditionalize the skip based on whether we are creating a
    new hunk header.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1437248 13f79535-47bb-0310-9956-ffa450edef68