Fix nested sessions' history saving #2307

Job	Run time
ruby-versions / Generate Ruby versions	3s
vterm-yamatanooroti 2.7 (latest reline)	1m 18s
vterm-yamatanooroti 2.7	1m 17s
vterm-yamatanooroti 3.0 (latest reline)	44s
vterm-yamatanooroti 3.0	45s
vterm-yamatanooroti 3.1 (latest reline)	50s
vterm-yamatanooroti 3.1	50s
vterm-yamatanooroti 3.2 (latest reline)	44s
vterm-yamatanooroti 3.2	52s
vterm-yamatanooroti head (latest reline)	43s
vterm-yamatanooroti head	48s
rake test 2.7 (latest reline)	42s
rake test 2.7	44s
rake test 3.0 (latest reline)	28s
rake test 3.0	30s
rake test 3.1 (latest reline)	30s
rake test 3.1	29s
rake test 3.2 (latest reline)	26s
rake test 3.2	30s
rake test head (latest reline)	29s
rake test head	35s
rake test truffleruby-head (latest reline)	7m 20s
rake test truffleruby-head	6m 44s
	28m 21s

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