Can't diff against the 00000000 revision

[Stefan Monnier <monnier@iro.umontreal.ca>]

If I look at the initial commit on a branch, I see something like:

    % git show d59cfff346c3e210adc26501f8cebf8da5ab2e7d
    commit d59cfff346c3e210adc26501f8cebf8da5ab2e7d
    Author: Stefan Monnier <monnier@iro.umontreal.ca>
    Date:   Wed Dec 2 20:46:51 2015 -0500
    
        Initial release
    
    diff --git a/bugit b/bugit
    new file mode 100755
    index 0000000..681bd38
    --- /dev/null
    +++ b/bugit
    @@ -0,0 +1,512 @@
    ...

which is great.  But I can't get the same result with

    git diff 0000000..681bd38

because it complains:

    % git diff 0000000..681bd38
    fatal: ambiguous argument '0000000..681bd38': unknown revision or path not in the working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'
    %

I bumped into this problem in a post-receive hook where I need to pay
attention to all newly added files, and where this problem means that
I can't use the same code for a newly added branch as for a push on
a pre-existing branch.

I currently work around the problem by adding a dummy empty branch, but
being able to use the revision 00000000 as a known reference to an empty
tree would come in really handy, and since it's already used at various
places in Git (post-receive hook and "git show" output, at least), it
would seem like a natural extension.


        Stefan