🐛 Fix `SequenceSet#merge` with another SequenceSet #1800

Job	Run time
ruby-versions / Generate Ruby versions	4s
build (3.1 / ubuntu-latest)	37s
build (3.1 / macos-latest)	24s
build (3.1 / windows-latest)	1m 30s
build (3.2 / ubuntu-latest)	41s
build (3.2 / macos-latest)	25s
build (3.2 / windows-latest)	2m 11s
build (3.3 / ubuntu-latest)	1m 13s
build (3.3 / macos-latest)	27s
build (3.3 / windows-latest)	1m 50s
build (3.4 / ubuntu-latest)	40s
build (3.4 / macos-latest)	24s
build (3.4 / windows-latest)	2m 20s
build (head / ubuntu-latest)	44s
build (head / macos-latest)	26s
build (head / windows-latest)	1m 26s
	15m 22s

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