object-store: rename and expand packed_git's sha1 member

This member is used to represent the pack checksum of the pack in
question.  Expand this member to be GIT_MAX_RAWSZ bytes in length so it
works with longer hashes and rename it to be "hash" instead of "sha1".
This transformation was made with a change to the definition and the
following semantic patch:

@@
struct packed_git *E1;
@@
- E1->sha1
+ E1->hash

@@
struct packed_git E1;
@@
- E1.sha1
+ E1.hash

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-walker.c

@@ -434,7 +434,7 @@ static int http_fetch_pack(struct walker *walker, struct alt_base *repo, unsigne
 
 	if (walker->get_verbosely) {
 		fprintf(stderr, "Getting pack %s\n",
-			sha1_to_hex(target->sha1));
+			sha1_to_hex(target->hash));
 		fprintf(stderr, " which contains %s\n",
 			sha1_to_hex(sha1));
 	}