Things to do:

- tidy up the code.  rediff.c has some nice routines that should be moved
  to diff.c.  More use of assert().
- interdiff: rewrite output_patch1_only in terms of calls to output_delta
  with a file handle open to /dev/null.
- Strip trailing CRs if necessary, like patch does.
- Handle RFC934 encapsulation, like patch does.
- Make interdiff support -I (?? don't remember what this is about).
- Teach rediff how to handle multiple added hunks.
- Add an option to make splitdiff separate out each hunk.