narrowspec: limit patterns to path: and rootfilesin: (BC) Some matcher patterns are computationally expensive and may even have security issues (e.g. evaluating some file sets). For these reasons, we want to limit the types of matcher patterns that can be used in narrow specs and by command line arguments used for defining narrow specs. This commit teaches ``narrowspec.parsepatterns()`` to validate the pattern types against "safe" patterns. Surprisingly, no existing tests broke. So tests for the feature have been added. We also added a function to validate a patterns data structure. This will be used in future commits. Differential Revision: https://phab.mercurial-scm.org/D4522