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.