dirstate.walk: fast path none-seen + match-always case for step 3
This case is a common one -- e.g. `hg diff`.
For a repository with 170,000 files, this speeds up perfstatus from 0.95
seconds to 0.88.
dirstate.walk: fast path match-always case during traversal
This case is a common one -- e.g. `hg status`.
For a repository with 170,000 files, this speeds up perfstatus --unknown from
2.15 seconds to 2.09.