commit-reach: add generalized find_reachable() #28291

Job	Run time
config	7s
fuzz smoke test	1m 15s
sparse	59s
win build	4m 5s
win+Meson build	4m 27s
rust-analysis	1m 32s
documentation	5m 7s
static-analysis	4m 14s
osx-reftable (macos-14)	19s
win+VS build	0s
osx-meson (macos-14)	33s
osx-clang (macos-14)	37s
osx-gcc (macos-14)	57s
linux-sha256 (ubuntu:rolling)	2m 18s
linux-breaking-changes (ubuntu:rolling)	2m 2s
debian-11 (debian:11)	2m 18s
linux-reftable-leaks (ubuntu:rolling)	1m 44s
fedora-breaking-changes-meson (fedora:latest)	1m 10s
linux-asan-ubsan (ubuntu:rolling)	2m 43s
linux-TEST-vars (ubuntu:20.04)	2m 1s
linux-meson (ubuntu:rolling)	1m 18s
win+VS test	0s
linux-musl-meson (alpine:latest)	50s
linux-leaks (ubuntu:rolling)	1m 45s
linux-reftable (ubuntu:rolling)	2m 20s
almalinux-8 (almalinux:8)	1m 48s
linux32 (i386/ubuntu:focal)	2m 13s
win test	-1s
win+Meson test	0s
	48m 41s

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