reftable: fix quadratic behavior when re-creating deleted refs #28869

Job	Run time
config	6s
fuzz smoke test	1m 22s
win build	5m 16s
win+Meson build	5m 55s
documentation	7m 24s
rust-analysis	1m 33s
sparse	3m 34s
static-analysis	6m 13s
osx-gcc (macos-14)	33m 1s
osx-meson (macos-14)	40m 55s
osx-reftable (macos-14)	30m 23s
linux-reftable-leaks (ubuntu:rolling)	59m 43s
debian-11 (debian:11)	25m 18s
linux-TEST-vars (ubuntu:20.04)	27m 20s
linux-reftable (ubuntu:rolling)	23m 58s
linux-breaking-changes (ubuntu:rolling)	27m 57s
linux-leaks (ubuntu:rolling)	58m 4s
osx-clang (macos-14)	37m 11s
linux-musl-meson (alpine:latest)	21m 15s
linux-meson (ubuntu:rolling)	23m 29s
linux32 (i386/ubuntu:20.04)	6h 0m 4s
fedora-breaking-changes-meson (fedora:latest)	19m 6s
linux-sha256 (ubuntu:rolling)	24m 47s
almalinux-8 (almalinux:8)	21m 3s
linux-asan-ubsan (ubuntu:rolling)	1h 10m 14s
win+VS build	-1s
win+VS test	-1s
win test (6)	44m 25s
win test (8)	4m 24s
win test (1)	7m 56s
win test (3)	10m 32s
win test (9)	6m 27s
win test (2)	6m 56s
win test (7)	7m 46s
win test (0)	9m 57s
win test (5)	10m 21s
win test (4)	1h 0m 34s
win+Meson test (0)	7m 42s
win+Meson test (1)	7m 13s
win+Meson test (8)	10m 22s
win+Meson test (5)	8m 45s
win+Meson test (7)	7m 35s
win+Meson test (3)	11m 17s
win+Meson test (4)	33m 56s
win+Meson test (6)	12m 57s
win+Meson test (9)	5m 12s
win+Meson test (2)	58m 56s
	21h 8m 22s

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