Reduce the amount of stat traffic generated by a walk. When we switched to the new walk code for commands, we no longer passed a list of specific files to the repo or dirstate walk or changes methods. This meant that we always walked and attempted to match everything, which was not efficient. Now, if we are given any patterns to match, or nothing at all, we still walk everything. But if we are given only file names that contain no glob characters, we only walk those.