restack optimization: git replay

git replay is an experimental new command that can replay commits on a new base.

https://git-scm.com/docs/git-replay/2.44.0

For rebases that apply cleanly (no conflicts), if this command is available, it'll be faster to restack because the working tree will never be touched.

And if we can pre-determine that there will be conflicts, we can support an option to perform a restack only for branches that'll restack cleanly.

Look for the restack command implementation in the codebase, likely in a cmd/restack or similar directory. Understand how git-spice currently performs rebases. Research git replay's API and integration points. Determine how to detect clean rebases and conflicts. The change involves adding a new git replay backend and possibly a flag for clean-only restacks.

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