reftable: fix quadratic behavior when re-creating deleted refs #28982

Job	Run time
config	9s
fuzz smoke test	2m 14s
win build	4m 52s
win+Meson build	4m 3s
sparse	3m 41s
rust-analysis	2m 27s
static-analysis	7m 8s
documentation	7m 54s
osx-meson (macos-14)	41m 26s
osx-gcc (macos-14)	37m 45s
osx-reftable (macos-14)	29m 9s
osx-clang (macos-14)	37m 38s
linux-musl-meson (alpine:latest)	19m 25s
linux-TEST-vars (ubuntu:20.04)	24m 4s
linux-asan-ubsan (ubuntu:rolling)	1h 11m 50s
linux-reftable-leaks (ubuntu:rolling)	1h 0m 39s
linux-reftable (ubuntu:rolling)	25m 18s
almalinux-8 (almalinux:8)	21m 10s
linux-sha256 (ubuntu:rolling)	25m 37s
linux-leaks (ubuntu:rolling)	58m 56s
debian-11 (debian:11)	22m 10s
linux-meson (ubuntu:rolling)	24m 51s
linux-breaking-changes (ubuntu:rolling)	22m 20s
fedora-breaking-changes-meson (fedora:latest)	23m 6s
linux32 (i386/ubuntu:20.04)	6h 0m 4s
win+VS build	0s
win+VS test	-1s
win+Meson test (8)	8m 58s
win+Meson test (9)	10m 36s
win+Meson test (2)	8m 12s
win+Meson test (3)	44m 37s
win+Meson test (5)	11m 31s
win+Meson test (0)	10m 42s
win+Meson test (7)	10m 30s
win+Meson test (1)	5m 38s
win+Meson test (4)	9m 34s
win+Meson test (6)	39m 23s
win test (6)	58m 17s
win test (8)	6m 40s
win test (7)	21m 13s
win test (2)	5m 50s
win test (1)	7m 12s
win test (3)	11m 25s
win test (4)	10m 4s
win test (9)	6m 21s
win test (5)	7m 17s
win test (0)	10m 22s
	20h 42m 17s

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