reftable: fix quadratic behavior when re-creating deleted refs #28952

Job	Run time
config	10s
fuzz smoke test	1m 29s
static-analysis	47m 25s
win build	5m 26s
sparse	3m 50s
documentation	7m 48s
win+Meson build	4m 14s
rust-analysis	2m 37s
osx-meson (macos-14)	42m 8s
osx-clang (macos-14)	36m 10s
osx-gcc (macos-14)	29m 32s
osx-reftable (macos-14)	29m 51s
linux-musl-meson (alpine:latest)	19m 4s
linux-sha256 (ubuntu:rolling)	25m 8s
linux32 (i386/ubuntu:20.04)	6h 0m 5s
win+VS build	0s
linux-reftable (ubuntu:rolling)	15m 49s
linux-meson (ubuntu:rolling)	24m 20s
debian-11 (debian:11)	22m 10s
linux-leaks (ubuntu:rolling)	59m 17s
fedora-breaking-changes-meson (fedora:latest)	19m 35s
linux-breaking-changes (ubuntu:rolling)	25m 25s
linux-reftable-leaks (ubuntu:rolling)	1h 0m 45s
linux-asan-ubsan (ubuntu:rolling)	1h 12m 22s
almalinux-8 (almalinux:8)	19m 47s
linux-TEST-vars (ubuntu:20.04)	15m 32s
win+VS test	0s
win+Meson test (4)	9m 35s
win+Meson test (5)	10m 56s
win+Meson test (8)	7m 36s
win+Meson test (6)	33m 23s
win+Meson test (3)	39m 54s
win+Meson test (2)	9m 1s
win+Meson test (9)	10m 32s
win+Meson test (1)	6m 27s
win+Meson test (7)	8m 56s
win+Meson test (0)	10m 54s
win test (4)	9m 28s
win test (7)	40m 19s
win test (6)	59m 14s
win test (5)	11m 21s
win test (3)	10m 53s
win test (2)	8m 26s
win test (9)	13m 37s
win test (8)	8m 14s
win test (0)	11m 14s
win test (1)	9m 0s
	21h 18m 59s

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