store: return just one filename in walk functions Various walk functions return `(revlog_type, decoded, encoded)` where decoded could be None. But no-one cares about `encoded` and expects `unencoded` to be present, except verify (because this can only happen with old repo formats). Simplify all this by either failing outright if a decoding a filename fails (instead of almost certainly failing with a type error due to treating None as a bytes), or skipping the filename but providing in an out argument for hg verify. Differential Revision: https://phab.mercurial-scm.org/D11248