[RFC] diff: add `diff.<driver>.process` for external hunk providers #28496

Job	Run time
config	7s
fuzz smoke test	1m 6s
static-analysis	5m 14s
win+Meson build	4m 8s
win build	3m 53s
sparse	3m 35s
osx-reftable (macos-14)	36m 5s
osx-meson (macos-14)	56m 33s
osx-gcc (macos-14)	1h 14m 38s
documentation	7m 42s
osx-clang (macos-14)	1h 10m 24s
rust-analysis	1m 35s
linux-meson (ubuntu:rolling)	24m 50s
linux-musl-meson (alpine:latest)	17m 58s
linux-breaking-changes (ubuntu:rolling)	28m 42s
linux-asan-ubsan (ubuntu:rolling)	1h 9m 35s
linux-leaks (ubuntu:rolling)	48m 32s
linux32 (i386/ubuntu:focal)	1h 26m 51s
linux-reftable (ubuntu:rolling)	17m 13s
linux-TEST-vars (ubuntu:20.04)	27m 40s
fedora-breaking-changes-meson (fedora:latest)	21m 14s
almalinux-8 (almalinux:8)	23m 32s
linux-reftable-leaks (ubuntu:rolling)	47m 25s
debian-11 (debian:11)	25m 41s
linux-sha256 (ubuntu:rolling)	22m 16s
win+VS build	0s
win+VS test	0s
win+Meson test (9)	8m 29s
win+Meson test (8)	9m 45s
win+Meson test (4)	11m 44s
win+Meson test (6)	13m 47s
win+Meson test (2)	8m 23s
win+Meson test (1)	8m 20s
win+Meson test (7)	8m 14s
win+Meson test (0)	7m 38s
win+Meson test (5)	11m 6s
win+Meson test (3)	21m 51s
win test (4)	8m 10s
win test (6)	11m 6s
win test (3)	12m 33s
win test (1)	6m 31s
win test (9)	8m 11s
win test (2)	5m 20s
win test (8)	8m 7s
win test (0)	9m 57s
win test (7)	8m 27s
win test (5)	13m 10s
	15h 27m 18s

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