add object_as_type helper for casting objects

When we call lookup_commit, lookup_tree, etc, the logic goes
something like:

  1. Look for an existing object struct. If we don't have
     one, allocate and return a new one.

  2. Double check that any object we have is the expected
     type (and complain and return NULL otherwise).

  3. Convert an object with type OBJ_NONE (from a prior
     call to lookup_unknown_object) to the expected type.

We can encapsulate steps 2 and 3 in a helper function which
checks whether we have the expected object type, converts
OBJ_NONE as appropriate, and returns the object.

Not only does this shorten the code, but it also provides
one central location for converting OBJ_NONE objects into
objects of other types. Future patches will use that to
enforce type-specific invariants.

Since this is a refactoring, we would want it to behave
exactly as the current code. It takes a little reasoning to
see that this is the case:

  - for lookup_{commit,tree,etc} functions, we are just
    pulling steps 2 and 3 into a function that does the same
    thing.

  - for the call in peel_object, we currently only do step 3
    (but we want to consolidate it with the others, as
    mentioned above). However, step 2 is a noop here, as the
    surrounding conditional makes sure we have OBJ_NONE
    (which we want to keep to avoid an extraneous call to
    sha1_object_info).

  - for the call in lookup_commit_reference_gently, we are
    currently doing step 2 but not step 3. However, step 3
    is a noop here. The object we got will have just come
    from deref_tag, which must have figured out the type for
    each object in order to know when to stop peeling.
    Therefore the type will never be OBJ_NONE.

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

blob.c

@@ -8,14 +8,7 @@ struct blob *lookup_blob(const unsigned char *sha1)
 	struct object *obj = lookup_object(sha1);
 	if (!obj)
 		return create_object(sha1, alloc_blob_node());
-	if (!obj->type)
-		obj->type = OBJ_BLOB;
-	if (obj->type != OBJ_BLOB) {
-		error("Object %s is a %s, not a blob",
-		      sha1_to_hex(sha1), typename(obj->type));
-		return NULL;
-	}
-	return (struct blob *) obj;
+	return object_as_type(obj, OBJ_BLOB, 0);
 }
 
 int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size)