Check-in [da7c8a19cd]

Update now follows the target's renames and saves undo state. A file outside the cone stays a record-only step: never written, chmodded, backed up or read for undo. A local edit that a rename would carry out of the cone is refused, naming the cone. check-in: 74aa290eda user: [email protected] tags: feat/submanifest-reader, main, trunk

# Conflicts: # internal/undo/undo.go check-in: d6ac19e580 user: [email protected] tags: feat/submanifest-reader, main, trunk

Update saved no undo state, so nothing could reverse it. It now reads the file of every step that writes or removes one before writing, as fossil's update calls undo_save, and saves those files with copies of vfile and vmerge and the checked-out version in the same transaction that moves the checkout to the target. If a write fails partway the files are put back, as fossil's undo_rollback does.

Checkout.Undo and Checkout.Redo (internal and public) swap that state back and forth and keep checkout-hash naming the restored version. The undo package gains ReadFile, SaveTx and RestoreFiles, and its undo table now has fossil's schema (isExe, unique pathname, rowid order, the executable bit read from disk), so fossil undo reverses a go-libfossil update and Checkout.Undo reverses fossil update; fossil 2.28 interop tests check both. Under js/wasm, which has no undo package, Update saves nothing.

Tracked in #446 check-in: 294a537880 user: [email protected] tags: feat/submanifest-reader, main, trunk

# Conflicts: # CHANGELOG.md check-in: 5d0a52d706 user: [email protected] tags: feat/submanifest-reader, main, trunk