Improve GitInfo for reading packed refs
GitInfo only reads branches in files /refs/heads/*; it should also
read those in /.git/packed-refs.
There are 4 tests in this commit to test this new "feature" and add some
code coverage to other methods. Given this class tests Git behaviour and
MediaWiki itself is versioned with Git, the test data are created during
the tests and removed at the end to avoid interference with Git as CVS.