repo: remove `MutableRepo::rebase_descendants_return_map` #422

Job	Run time
Build binary artifacts (linux-x86_64-musl)	4m 58s
Build binary artifacts (linux-x86_64-gnu)	5m 14s
Build binary artifacts (linux-aarch64-musl)	5m 28s
Build binary artifacts (linux-aarch64-gnu)	5m 14s
Build binary artifacts (macos-x86_64)	7m 29s
Build binary artifacts (macos-aarch64)	6m 8s
Build binary artifacts (win-x86_64)	8m 32s
	43m 3s

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