trunk/8d19db378e5c69983e46b2974433668fcfe1bdd6: [GHF] Read and validate GitHub-native PR stacks (#199713)

发生了什么

Impact: none for users: trymerge.py only takes the Pull Request resolved prefix and the ghstack head-ref regex from the new native_stack.py , and nothing calls the stack functions before the next commits Risk: low What native_stack.py reads a PR's GitHub-native stack with one GraphQL query, finds what landed on the default branch from the commits' own trailers, and decides whether merging one PR of the stack is safe…

摘要按规则整理自下方来源原文

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