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dirstate: optimize walk() by using match.visitdir()

We already have the logic for restricting directory walks in
match.visitdir() that we use for treemanifests. We should take
advantage of it when walking the working copy as well.

This speeds up "hg st -I rootfilesin:." on the Firefox repo from
0.587s to 0.305s on warm disk (and much more on cold disk). More time
is spent reading the dirstate than walking the working copy after.

I tried to find scenarios where calling match.visitdir() would be a
noticeable overhead, but I couldn't find any. I encourage the reader
to try for themselves, since this is performance-critical code.

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