treemanifest: optimize treemanifest._walk() to skip directories This makes treemanifest.walk() not visit submanifests that are known not to have any matching files. It does this by calling match.visitdir() on submanifests as it walks. This change also updates largefiles to be able to work with this new behavior in treemanifests. It overrides match.visitdir(), the function that dictates how walk() and matches() skip over directories. The greatest speed improvements are seen with narrower scopes. For example, this commit speeds up the following command on the Mozilla repo from 1.14s to 1.02s: hg files -r . dom/apps/ Whereas with a wider scope, dom/, the speed only improves from 1.21s to 1.13s. As with similar a similar optimization to treemanifest.matches(), this change will bring out even bigger performance improvements once treemanifests are loaded lazily. Once that happens, we won't just skip over looking at submanifests, but we'll skip even loading them.