treemanifest: refactor treemanifest.walk() This refactor is a preparation for an optimization in the next commit. This introduces a recursive element that recurses each submanifest. By using a recursive function, the next commit can avoid walking over some subdirectories altogether.

manifest: move changectx.walk() to manifests The logic of walking a manifest to yield files matching a match object is currently being done by context, not the manifest itself. This moves the walk() function to both manifestdict and treemanifest. This separate implementation will also permit differing, optimized implementations for each manifest.

subrepo: precisely identify the missing subrepo spec file It isn't obvious which file is the problem with deep subrepos, so provide the path. Since the parsing is done with a ctx and not a subrepo object, it isn't possible to display a path from the root subrepo. Therefore, the path shown is relative to cwd. There's no test coverage for the first abort, and I couldn't figure out how to trigger it, but it is changed for consistency.