tests/narrow-library.sh

typing: add type hints to the `opener` attributes and arguments of revlog

When making revlog and filelog classes visible to pytype, it got confused quite
a bit in `mercurial/revlogutils/rewrite.py`, thinking it had a plain `Callable`,
and flagging additional methods on it like `join()` and `rename()`.  I couldn't
figure out how it reduced to that (and PyCharm flagged `opener` references as
`Any`), but this makes it happy.  So make this change before making the classes
visible.

The vfs class hierarchy is a bit wonky (e.g. `filteredvfs` is not a `vfs`), so
this may need to be revisited with a Protocol class that covers all of the `vfs`
classes.  But for now, everything works.

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[experimental]
changegroup3 = True
EOF