Fix nested sessions' history saving #2282

Job	Run time
ruby-versions / Generate Ruby versions	2s
rake test 2.7 (latest reline)	53s
rake test 2.7	48s
rake test 3.0 (latest reline)	33s
rake test 3.0	50s
rake test 3.1 (latest reline)	29s
rake test 3.1	30s
rake test 3.2 (latest reline)	26s
rake test 3.2	26s
rake test head (latest reline)	28s
rake test head	30s
rake test truffleruby-head (latest reline)	7m 14s
rake test truffleruby-head	7m 59s
vterm-yamatanooroti 2.7 (latest reline)	1m 15s
vterm-yamatanooroti 2.7	1m 18s
vterm-yamatanooroti 3.0 (latest reline)	42s
vterm-yamatanooroti 3.0	42s
vterm-yamatanooroti 3.1 (latest reline)	2m 2s
vterm-yamatanooroti 3.1	1m 54s
vterm-yamatanooroti 3.2 (latest reline)	59s
vterm-yamatanooroti 3.2	48s
vterm-yamatanooroti head (latest reline)	55s
vterm-yamatanooroti head	1m 39s
	33m 22s

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