commit-graph: fix topo_levels slab propagation regression #28872

Job	Run time
config	22s
fuzz smoke test	1m 26s
rust-analysis	4m 22s
static-analysis	4m 31s
win build	4m 22s
osx-clang (macos-14)	4m 22s
osx-meson (macos-14)	4m 22s
osx-reftable (macos-14)	4m 22s
sparse	4m 22s
documentation	4m 22s
osx-gcc (macos-14)	4m 22s
linux-asan-ubsan (ubuntu:rolling)	4m 22s
debian-11 (debian:11)	4m 22s
linux-sha256 (ubuntu:rolling)	4m 22s
linux-musl-meson (alpine:latest)	4m 22s
linux-reftable-leaks (ubuntu:rolling)	4m 22s
linux-leaks (ubuntu:rolling)	4m 22s
linux-meson (ubuntu:rolling)	4m 22s
linux-breaking-changes (ubuntu:rolling)	4m 22s
fedora-breaking-changes-meson (fedora:latest)	4m 22s
win+Meson build	4m 22s
linux-reftable (ubuntu:rolling)	4m 22s
linux-TEST-vars (ubuntu:20.04)	4m 22s
linux32 (i386/ubuntu:20.04)	4m 22s
almalinux-8 (almalinux:8)	4m 22s
win+VS test	0s
win+VS build	0s
win+Meson test	0s
win test	0s
	1h 42m 23s

Provide feedback