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

mercurial/context.py

@@ -2129,7 +2129,7 b' class overlayworkingfilectx(workingfilec'
     def cmp(self, fctx):
         return self.data() != fctx.data()
 
-    def ctx(self):
+    def changectx(self):
         return self._parent
 
     def data(self):

mercurial/filemerge.py

@@ -469,7 +469,7 b' def _idump(repo, mynode, orig, fcd, fco,'
     # calls might be depending on.
     from . import context
     if isinstance(fcd, context.overlayworkingfilectx):
-        fcd.ctx().flushall()
+        fcd.changectx().flushall()
 
     util.writefile(a + ".local", fcd.decodeddata())
     repo.wwrite(fd + ".other", fco.data(), fco.flags())