Don't require new revisions on empty commits.

Don't require new revisions on empty commits.

For example, hg can't:
  $ mkdir x; cd x;
  x$ hg init;
  x$ echo a> b; hg add b;
  x$ hg commit -m 'r1';
  x$ hg commit -m 'r2';
  nothing changed
  x$ hg log;
  changeset:   0:e30558c36fca
  tag:         tip
  user:        W. Trevor King <wking@drexel.edu>
  date:        Sun Dec 13 19:48:47 2009 -0500
  summary:     hi
  x$ cd ..; rm -rf x

We shouldn't need this functionality anyway ;).