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