filter: add a comment so that people do not forget to update subsettable
Changeset 175c6fd8cacc moved `subsettable` from `mercurial/repoview.py` to
`mercurial/branchmap.py`. This mean that `filtertable` and `subsettable` are no
longer next to each other. So we add a comment to remind people to update both.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.