doc: Add a explanation of Git's data model #25235

Job	Run time
config	8s
fuzz smoke test	1m 16s
win+VS build	0s
win build	4m 47s
osx-clang (macos-13)	1h 41m 48s
osx-reftable (macos-13)	1s
static-analysis	5m 22s
rust-analysis	1m 37s
osx-meson (macos-13)	33m 55s
osx-gcc (macos-13)	1s
win+Meson build	5m 33s
sparse	3m 37s
documentation	4m 58s
linux-meson (ubuntu:rolling)	12m 3s
linux-leaks (ubuntu:rolling)	6m 20s
linux32 (i386/ubuntu:focal)	10m 8s
linux-musl-meson (alpine:latest)	8m 19s
linux-reftable (ubuntu:rolling)	5m 22s
almalinux-8 (almalinux:8)	10m 7s
debian-11 (debian:11)	9m 51s
linux-sha256 (ubuntu:rolling)	13m 18s
fedora-breaking-changes-meson (fedora:latest)	9m 36s
linux-reftable-leaks (ubuntu:rolling)	7m 42s
linux-asan-ubsan (ubuntu:rolling)	2m 49s
linux-TEST-vars (ubuntu:20.04)	13m 8s
linux-breaking-changes (ubuntu:rolling)	12m 52s
win+VS test	-1s
win test (0)	7m 21s
win test (2)	8m 38s
win test (3)	6m 22s
win test (7)	6m 12s
win test (4)	10m 21s
win test (9)	6m 50s
win test (5)	8m 10s
win test (1)	6m 25s
win test (8)	10m 13s
win test (6)	7m 53s
win+Meson test (0)	7m 34s
win+Meson test (1)	11m 25s
win+Meson test (2)	9m 49s
win+Meson test (9)	11m 44s
win+Meson test (8)	7m 34s
win+Meson test (4)	12m 38s
win+Meson test (5)	11m 0s
win+Meson test (7)	8m 56s
win+Meson test (6)	6m 56s
win+Meson test (3)	11m 29s
	7h 42m 7s

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