object-file: inline empty tree and blob literals

object-file: inline empty tree and blob literals

We define macros with the bytes of the empty trees and blobs for sha1
and sha256. But since e1ccd7e2b1 (sha1_file: only expose empty object
constants through git_hash_algo, 2018-05-02), those are used only for
initializing the git_hash_algo entries. Any other code using the macros
directly would be suspicious, since a hash_algo pointer is the level of
indirection we use to make everything work with both sha1 and sha256.

So let's future proof against code doing the wrong thing by dropping the
macros entirely and just initializing the structs directly.

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