localrepo: iterate over manifest key/value pairs in status
This saves us a couple of dict lookups in the common case, and improves
the performance of the status method by 5% (measured with util.timed)
in a repo with a large manifest.
match: more accurately report when we're always going to match
This improves the performance of log --patch and --stat by about
20% for moderately large manifests (e.g. mozilla-central) for the
common case of no -I/-X patterns.