Merge branch 'mm/diff-process-hunks' into seen

A new `diff.<driver>.process` configuration has been introduced to
allow a long-running external process to act as a hunk provider to
allows external tools to control which lines Git considers changed
while leaving all output formatting (word diff, color, blame, etc.) to
Git's standard pipeline.

* mm/diff-process-hunks:
  blame: consult diff process for no-hunk detection
  diff: bypass diff process with --no-ext-diff and in format-patch
  diff: add long-running diff process via diff.<driver>.process
  sub-process: separate process lifecycle from hashmap management
  userdiff: add diff.<driver>.process config
  xdiff: support external hunks via xpparam_t

diff --cc Documentation/diff-options.adoc
Simple merge

diff --cc Makefile
Simple merge

diff --cc diff.c
Simple merge

diff --cc meson.build
Simple merge

diff --cc t/meson.build
Simple merge