match: handle everything-matching using new alwaysmatcher Having a special matcher that always matches seems to make more sense than making one of the other matchers handle the case. For now, we just use this new matcher when no patterns were provided.

match: add __repr__ for subdirmatcher Should at least be useful for debugging. Would matter for correctness too if fsmonitor or Facebook's sparse extension worked with subrepos (which I don't know if they do).

tests: make test-manifest.py portable to Python 3 Lots of b prefixes here, and https://bugs.python.org/issue29714 means that this test is still very broken on Python 3.6 and 3.6.1, but 3.6.2 should things (based on testing using tip of the 3.6 branch from git). #cleanup-only