[RFC] diff: add `diff.<driver>.process` for external hunk providers #28061

Job	Run time
config	9s
fuzz smoke test	2m 21s
win build	4m 48s
win+Meson build	4m 5s
sparse	3m 40s
documentation	7m 0s
osx-reftable (macos-14)	52s
osx-clang (macos-14)	6m 50s
linux-musl-meson (alpine:latest)	9m 37s
rust-analysis	1m 41s
osx-gcc (macos-14)	3m 21s
osx-meson (macos-14)	16m 6s
linux-reftable (ubuntu:rolling)	8m 28s
linux-meson (ubuntu:rolling)	5m 31s
debian-11 (debian:11)	7m 21s
linux-reftable-leaks (ubuntu:rolling)	8m 20s
win+VS build	0s
linux-TEST-vars (ubuntu:20.04)	6m 23s
almalinux-8 (almalinux:8)	6m 45s
fedora-breaking-changes-meson (fedora:latest)	2m 6s
static-analysis	6m 14s
linux-breaking-changes (ubuntu:rolling)	2m 57s
linux-sha256 (ubuntu:rolling)	6m 20s
linux-leaks (ubuntu:rolling)	5m 28s
linux32 (i386/ubuntu:focal)	5m 27s
win+VS test	-1s
linux-asan-ubsan (ubuntu:rolling)	2m 54s
win test (7)	7m 54s
win test (0)	9m 31s
win test (4)	13m 5s
win test (3)	9m 8s
win test (5)	13m 6s
win test (8)	7m 45s
win test (6)	9m 27s
win test (2)	11m 14s
win test (9)	6m 6s
win test (1)	7m 39s
win+Meson test (7)	10m 0s
win+Meson test (3)	4m 2s
win+Meson test (2)	10m 2s
win+Meson test (6)	9m 9s
win+Meson test (5)	8m 17s
win+Meson test (0)	7m 46s
win+Meson test (8)	7m 2s
win+Meson test (4)	5m 29s
win+Meson test (1)	8m 11s
win+Meson test (9)	4m 29s
	5h 4m 5s

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