index-pack: retain child bases in delta cache #28427

Job	Run time
config	6s
rust-analysis	-1s
${{ matrix.vector.jobname }} (${{ matrix.vector.image }})	-1s
fuzz smoke test	-1s
documentation	-1s
static-analysis	-1s
win+Meson build	-1s
${{ matrix.vector.jobname }} (${{ matrix.vector.pool }})	-1s
win build	-1s
win+VS build	-1s
win test	0s
sparse	-1s
win+Meson test	0s
win+VS test	0s
	-4s

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