index-pack: distinguish missing objects from type errors

When we fetch a pack that does not contain an object we
expected to receive, we get an error like:

  $ git init --bare tmp.git && cd tmp.git
  $ git fetch ../parent.git
  [...]
  error: Could not read 964953ec7bcc0245cb1d0db4095455edd21a2f2e
  fatal: Failed to traverse parents of commit b8247b40caf6704fe52736cdece6d6aae87471aa
  error: ../parent.git did not send all necessary objects

This comes from the check_everything_connected rev-list. If
we try cloning the same repo (rather than a fetch), we end
up using index-pack's --check-self-contained-and-connected
option instead, which produces output like:

  $ git clone --no-local --bare parent.git tmp.git
  [...]
  fatal: object of unexpected type
  fatal: index-pack failed

Not only is the sha1 missing, but it's a misleading message.
There's no type problem, but rather a missing object
problem; we don't notice the difference because we simply
compare OBJ_BAD != OBJ_BLOB.  Let's provide a different
message for this case:

  $ git clone --no-local --bare parent.git tmp.git
  fatal: did not receive expected object 6b00a8c61ed379d5f925a72c1987c9c52129d364
  fatal: index-pack failed

While we're at it, let's also improve a true type mismatch
error to look like

  fatal: object 6b00a8c61ed379d5f925a72c1987c9c52129d364: expected type blob, got tree
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/index-pack.c

@@ -200,8 +200,13 @@ static unsigned check_object(struct object *obj)
 	if (!(obj->flags & FLAG_CHECKED)) {
 		unsigned long size;
 		int type = sha1_object_info(obj->sha1, &size);
-		if (type != obj->type || type <= 0)
-			die(_("object of unexpected type"));
+		if (type <= 0)
+			die(_("did not receive expected object %s"),
+			      sha1_to_hex(obj->sha1));
+		if (type != obj->type)
+			die(_("object %s: expected type %s, found %s"),
+			    sha1_to_hex(obj->sha1),
+			    typename(obj->type), typename(type));
 		obj->flags |= FLAG_CHECKED;
 		return 1;
 	}