hard-code the empty tree object
hard-code the empty tree object

Now any commands may reference the empty tree object by its
sha1 (4b825dc642cb6eb9a060e54bf8d69288fbee4904). This is
useful for showing some diffs, especially for initial
commits.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git