Fix nested sessions' history saving #2312

Job	Run time
ruby-versions / Generate Ruby versions	2s
rake test 2.7 (latest reline)	44s
rake test 2.7	57s
rake test 3.0 (latest reline)	27s
rake test 3.0	34s
rake test 3.1 (latest reline)	36s
rake test 3.1	35s
rake test 3.2 (latest reline)	32s
rake test 3.2	30s
rake test head (latest reline)	38s
rake test head	33s
rake test truffleruby-head (latest reline)	6m 29s
rake test truffleruby-head	7m 27s
vterm-yamatanooroti 2.7 (latest reline)	1m 14s
vterm-yamatanooroti 2.7	1m 11s
vterm-yamatanooroti 3.0 (latest reline)	45s
vterm-yamatanooroti 3.0	41s
vterm-yamatanooroti 3.1 (latest reline)	59s
vterm-yamatanooroti 3.1	55s
vterm-yamatanooroti 3.2 (latest reline)	48s
vterm-yamatanooroti 3.2	43s
vterm-yamatanooroti head (latest reline)	49s
vterm-yamatanooroti head	53s
	29m 2s

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