Significant speed-up of datevec

[John W. Eaton]

On 18-Dec-2008, Jaroslav Hajek wrote:

| I applied this patch. If you intend to make more than a few
| contributions to Octave, please consider making yourself familiar with
| Mercurial and then using it to create, organize and send patches.

These latest two changes should have ChangeLog entries.

Also, what is the purpose of this change in unique.m?

  -  ## I don't know why anyone would need reverse indices, but it
  -  ## was an interesting challenge.  I welcome cleaner solutions.
     if (nargout >= 3)
       j = i;
  -    j(i) = cumsum (prepad (! match, n, 1));
  +    j(i) = cumsum ([1 !match]);


Also, if it is not already in the the coding standards, I think we
should add something saying that elements in matrices, cell arrays,
and lists of values returned from functions should be separated by
commas (or semicolons) not just whitespace.

jwe