reftable: fix quadratic behavior when re-creating deleted refs #28867

Job	Run time
config	11s
fuzz smoke test	1m 32s
win build	28s
win+Meson build	28s
rust-analysis	22s
static-analysis	35s
sparse	35s
documentation	28s
osx-reftable (macos-14)	28s
osx-gcc (macos-14)	28s
osx-clang (macos-14)	28s
osx-meson (macos-14)	28s
linux-TEST-vars (ubuntu:20.04)	28s
linux-leaks (ubuntu:rolling)	28s
linux-reftable-leaks (ubuntu:rolling)	28s
debian-11 (debian:11)	28s
linux-asan-ubsan (ubuntu:rolling)	28s
linux-meson (ubuntu:rolling)	28s
fedora-breaking-changes-meson (fedora:latest)	28s
linux-musl-meson (alpine:latest)	28s
linux-sha256 (ubuntu:rolling)	28s
linux-breaking-changes (ubuntu:rolling)	28s
linux-reftable (ubuntu:rolling)	28s
linux32 (i386/ubuntu:20.04)	28s
almalinux-8 (almalinux:8)	28s
win+VS build	0s
win+VS test	0s
win+Meson test	0s
win test	0s
	12m 35s

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