patch-id: use stable patch-id for rebases

patch-id: use stable patch-id for rebases

Git doesn't persist patch-ids during the rebase process, so there is
no need to specifically invoke the unstable variant. Use the stable
logic for all internal patch-id calculations to minimize the number of
code paths and improve test coverage.

Signed-off-by: Jerry Zhang <redacted>
Signed-off-by: Junio C Hamano <redacted>