Simplify old text-delta code with new convenience function.
Simplify old text-delta code with new convenience function.

* libsvn_delta/text-delta.c (svn_txdelta_next_window): Use
  compute_window() once we have the window metadata.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion