reftable: fix quadratic behavior when re-creating deleted refs #28804

Job	Run time
config	8s
fuzz smoke test	1m 17s
win+Meson build	4m 23s
win build	4m 44s
documentation	7m 22s
static-analysis	6m 33s
win+VS build	0s
osx-gcc (macos-14)	42m 27s
rust-analysis	1m 27s
osx-clang (macos-14)	36m 36s
sparse	56m 19s
osx-reftable (macos-14)	30m 45s
osx-meson (macos-14)	47m 25s
linux-musl-meson (alpine:latest)	17m 43s
linux-asan-ubsan (ubuntu:rolling)	40m 47s
almalinux-8 (almalinux:8)	14m 54s
linux-breaking-changes (ubuntu:rolling)	23m 25s
debian-11 (debian:11)	20m 38s
fedora-breaking-changes-meson (fedora:latest)	18m 4s
linux-leaks (ubuntu:rolling)	41m 41s
linux-meson (ubuntu:rolling)	23m 42s
linux-sha256 (ubuntu:rolling)	27m 14s
linux-reftable-leaks (ubuntu:rolling)	42m 55s
linux-reftable (ubuntu:rolling)	23m 37s
linux-TEST-vars (ubuntu:20.04)	26m 22s
linux32 (i386/ubuntu:20.04)	6h 0m 2s
win+VS test	-1s
win+Meson test (5)	8m 37s
win+Meson test (2)	46m 48s
win+Meson test (3)	11m 21s
win+Meson test (6)	13m 26s
win+Meson test (4)	21m 54s
win+Meson test (1)	6m 57s
win+Meson test (8)	9m 32s
win+Meson test (0)	8m 52s
win+Meson test (7)	7m 14s
win+Meson test (9)	6m 45s
win test (2)	7m 14s
win test (3)	11m 17s
win test (0)	11m 0s
win test (4)	59m 50s
win test (8)	6m 58s
win test (1)	9m 7s
win test (6)	21m 18s
win test (7)	8m 1s
win test (5)	9m 35s
win test (9)	5m 48s
	20h 12m 3s

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