filecontext: use 'is not None' to check for filelog existence Previously we used 'if filelog:' to check if the filelog existed. If the instance did exist, this pattern then calls len() on the filelog to see if it is empty. I'm developing a filelog replacement that doesn't have len() implemented, so it's better to do an explicit 'is not None' check here instead. Also change _changeid() to return the _changeid attribute if it has it. Previously it would try to obtain it from the _changectx(), and if that did not exist it would construct the _changectx() using the linkrev. In the extension I'm working on, filectx's don't have easy access to linkrevs so avoiding this when possible is better.