Make git-blame fail when working tree is needed and we're not in one

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-blame.c

@@ -2342,6 +2342,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		 * do not default to HEAD, but use the working tree
 		 * or "--contents".
 		 */
+		setup_work_tree();
 		sb.final = fake_working_tree_commit(path, contents_from);
 		add_pending_object(&revs, &(sb.final->object), ":");
 	}