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Patch id changes if users reorder file diffs that make up a patch. As the result is functionally equivalent, a different patch id is surprising to many users. In particular, reordering files using diff -O is helpful to make patches more readable (e.g. API header diff before implementation diff). Add an option to change patch-id behaviour making it stable against these kinds of patch change: calculate SHA1 hash for each hunk separately and sum all hashes (using a symmetrical sum) to get patch id We use a 20byte sum and not xor - since xor would give 0 output for patches that have two identical diffs, which isn't all that unlikely (e.g. append the same line in two places). The new behaviour is enabled - when patchid.stable is true - when --stable flag is present Using a new flag --unstable or setting patchid.stable to false force the historical behaviour. In the documentation, clarify that patch ID can now be a sum of hashes, not a hash. Document how command line and config options affect the behaviour. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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