doc: add an explanation of Git's data model #25196

Job	Run time
config	8s
fuzz smoke test	1m 38s
win+VS build	0s
win build	5m 17s
documentation	29m 4s
win+Meson build	3m 47s
osx-clang (macos-13)	38m 14s
osx-reftable (macos-13)	43m 31s
osx-meson (macos-13)	30m 45s
osx-gcc (macos-13)	27m 5s
static-analysis	5m 23s
sparse	3m 37s
almalinux-8 (almalinux:8)	9m 31s
debian-11 (debian:11)	10m 3s
linux-sha256 (ubuntu:rolling)	1h 11m 3s
linux-breaking-changes (ubuntu:rolling)	17m 54s
linux-reftable-leaks (ubuntu:rolling)	6m 55s
linux-leaks (ubuntu:rolling)	9m 22s
linux-asan-ubsan (ubuntu:rolling)	10m 57s
linux-reftable (ubuntu:rolling)	7m 50s
pedantic (fedora:latest)	1m 47s
linux-musl-meson (alpine:latest)	8m 3s
linux-meson (ubuntu:rolling)	5m 0s
win+VS test	-1s
linux-TEST-vars (ubuntu:20.04)	3m 2s
linux32 (i386/ubuntu:focal)	7m 56s
win test (0)	5m 51s
win test (1)	8m 42s
win test (2)	6m 42s
win test (9)	6m 48s
win test (7)	6m 30s
win test (3)	7m 13s
win test (4)	9m 51s
win test (5)	6m 58s
win test (6)	6m 20s
win test (8)	10m 39s
win+Meson test (8)	7m 48s
win+Meson test (0)	10m 42s
win+Meson test (9)	11m 25s
win+Meson test (3)	10m 11s
win+Meson test (4)	10m 47s
win+Meson test (6)	8m 36s
win+Meson test (7)	10m 41s
win+Meson test (1)	13m 2s
win+Meson test (2)	12m 21s
win+Meson test (5)	9m 0s
	8h 57m 58s

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