[RFC] diff: add `diff.<driver>.process` for external hunk providers #28498

Job	Run time
config	7s
fuzz smoke test	1m 1s
win+Meson build	4m 35s
win build	5m 25s
sparse	3m 40s
rust-analysis	1m 33s
documentation	7m 8s
static-analysis	5m 44s
osx-reftable (macos-14)	28m 43s
osx-meson (macos-14)	55m 9s
almalinux-8 (almalinux:8)	19m 54s
linux-leaks (ubuntu:rolling)	49m 27s
linux-sha256 (ubuntu:rolling)	28m 43s
linux-TEST-vars (ubuntu:20.04)	23m 55s
osx-clang (macos-14)	43m 49s
linux-breaking-changes (ubuntu:rolling)	28m 42s
fedora-breaking-changes-meson (fedora:latest)	21m 22s
linux-asan-ubsan (ubuntu:rolling)	1h 12m 0s
debian-11 (debian:11)	22m 32s
linux-musl-meson (alpine:latest)	23m 47s
linux-reftable-leaks (ubuntu:rolling)	47m 50s
linux-reftable (ubuntu:rolling)	17m 8s
osx-gcc (macos-14)	6h 0m 18s
linux-meson (ubuntu:rolling)	25m 44s
linux32 (i386/ubuntu:focal)	6h 0m 4s
win+VS build	0s
win+VS test	0s
win+Meson test (6)	12m 9s
win+Meson test (2)	8m 29s
win+Meson test (7)	9m 9s
win+Meson test (9)	7m 45s
win+Meson test (8)	9m 10s
win+Meson test (1)	7m 57s
win+Meson test (3)	1h 1m 51s
win+Meson test (4)	11m 44s
win+Meson test (0)	5m 17s
win+Meson test (5)	10m 54s
win test (6)	10m 43s
win test (4)	7m 53s
win test (7)	8m 15s
win test (0)	9m 59s
win test (8)	7m 46s
win test (9)	6m 44s
win test (5)	57m 28s
win test (1)	7m 17s
win test (3)	9m 38s
win test (2)	5m 41s
	1d 1h 34m 9s

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