dirstate-v2: Store a hash of ignore patterns (.hgignore) Later, this help extend `read_dir` caching to directories that contain ignored files (but no unknown files). Such cache must be invalidated when ignore patterns change since a formerly-ignored file might become unknown. This helps the default configuration of `hg status` where unknown files must be listed, but ignored files are not. Differential Revision: https://phab.mercurial-scm.org/D10836