revlog: list older-but-still-around file in `files`
These older files will eventually be removed, but we have to take them in account until they are. So we update `files` to keep fncache happy.
Differential Revision: https://phab.mercurial-scm.org/D10868
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.