Add several uses of get_object_hash.

Convert most instances where the sha1 member of struct object is
dereferenced to use get_object_hash.  Most instances that are passed to
functions that have versions taking struct object_id, such as
get_sha1_hex/get_oid_hex, or instances that can be trivially converted
to use struct object_id instead, are not converted.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>

combine-diff.c

@@ -1540,9 +1540,9 @@ void diff_tree_combined_merge(const struct commit *commit, int dense,
 	struct sha1_array parents = SHA1_ARRAY_INIT;
 
 	while (parent) {
-		sha1_array_append(&parents, parent->item->object.sha1);
+		sha1_array_append(&parents, get_object_hash(parent->item->object));
 		parent = parent->next;
 	}
-	diff_tree_combined(commit->object.sha1, &parents, dense, rev);
+	diff_tree_combined(get_object_hash(commit->object), &parents, dense, rev);
 	sha1_array_clear(&parents);
 }