Don't mistake a refetched repo for an externally modified one (#31108) ### Description After `bazel fetch --force` (or any other command that refetches a repo without loading its packages), the next `bazel build` warned that a file of the repo "has been modified externally" and fetched the repo a second time. The check for external modifications of repo files scanned all `FileStateValue` nodes in the graph via `MemoizingEvaluator#getValues()`, which also returns the last values of nodes that are already dirty. Refetching a repo re-evaluates its `RepositoryDirectoryValue`, which is not comparable and thus dirties the file state nodes of all files in the repo. A build re-evaluates these nodes when it loads the affected packages, but `fetch` doesn't request them and, unlike `BuildTool`, also doesn't delete dirty nodes at the end of the command. The stale nodes then survived until the next build, where the `ExternalDirtinessChecker` compared their old contents proxies with the rewritten files, reported an external modification and deleted the marker file to force another fetch. Only scan nodes that are done: dirty nodes are re-evaluated when they are next requested anyway, so their last values can't be used to detect changes that Skyframe doesn't already know about. ### Motivation Every `bazel fetch --force` was followed by a second fetch of the same repos in the next build, together with a misleading warning about external modifications. ### Build API Changes No ### Release Notes RELNOTES: `bazel fetch --force` no longer causes the fetched repos to be fetched again by the next build with a spurious warning about external modifications. Closes #31108 COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/bazel/pull/31108 from fmeum:fix-stale-external-repo-file-check 739a90bd6f1b0d3d1d9f2ad88078fca2762fe2bf PiperOrigin-RevId: 981245657 Change-Id: I98bc3c1162e55361ca435823c749f25759a0469f
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