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
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