replaceShortenedKeysInBranch: less re-allocs #6957

Job	Run time
test-hive (engine, withdrawals, 0)	6m 44s
test-hive (engine, cancun, 3)	9m 1s
test-hive (engine, auth, 0)	3m 45s
test-hive (engine, exchange-capabilities, 0)	3m 46s
test-hive (engine, api, 0)	7m 4s
test-hive (rpc, compat, 23)	3m 3s
	33m 23s

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