Use $Id$ as the ident attribute keyword rather than $ident$ to be consistent with...

Use $Id$ as the ident attribute keyword rather than $ident$ to be consistent with other VCSs

$Id$ is present already in SVN and CVS; it would mean that people
converting their existing repositories won't have to make any changes to
the source files should they want to make use of the ident attribute.

Given that it's a feature that's meant to calm those very people, it
seems obtuse to make them edit every file just to make use of it.

I think that bzr uses $Id$; Mercurial has examples hooks for $Id$;
monotone has $Id$ on its wishlist.  I can't think of a good reason not
to stick with the de-facto standard and call ours $Id$ instead of
$ident$.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>