What: a "Behaviour changes on the git path" section in the source-update developer guide: ZIP
fallback only from an untouched checkout, arm failure stops before any move, startup syntax
judged at the target by an admitted interpreter in one cat-file spawn, moves by commit id with
HEAD verified, the upstream ff judged before it moves, and a gone-target marker kept over a
dirty tree.
Why: the reviewer asked which of these are intended; each is, and release notes need the list.
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