context: switch ctx() use to changectx() I added `ctx()` to `overlayworkingfilectx`, (and before that, `absentfilectx`), because `absentfilectx` had reference to this function in its `cmp()` function. But the standard is actually `changectx()`, and no other class implements `ctx()`. So let's use the standard name. (As a result, I'm not sure that part of the `absentfilectx` comparator ever worked! It was written before I added either function.) This will be necessary in the next patch. Differential Revision: https://phab.mercurial-scm.org/D1211