tests: add test coverage for lfdirstate invalidation of linear update
f72d73937853 introduced a significant performance regression: All largefiles
are marked 'normallookup' in lfdirstate by linear (or noop) updates and has to
be rehashed by the next command.
To avoid such regressions, keep an eye on the dirstate content after a plain
'hg up'.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.