memfilectx: changectx argument is not mandatory
As changectx argument is now mandatory for memfilectx, add a compatibility
layer to pass it when necessary.
The commit that made the parament mandatory in core is 8a0cac20a1ad
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes