localrepo: update branchcache in a more reliable way
test-mq-cache.t did apparently look at stale cache content.
Testing with different locking mechanism happened to update the cache more
frequently and thus caused a test failure.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.