[PATCH v2 0/3] diff: fix --exit-code with external diff

Changes since v1:
- old patch 1 was merged
- old patch 2 was merged and reverted
- new patch 1 adds tests that exercises diff --exit-code and --quiet
  together with all methods for specifying external diffs
- new patch 2 adds a struct to store the flag that patch 3 adds
- patch 3 adds configuration options and an environment variable to
  accept diff(1)-style exit codes from external diffs

  t4020: test exit code with external diffs
  userdiff: add and use struct external_diff
  diff: let external diffs report that changes are uninteresting

 Documentation/config/diff.txt   | 14 ++++++++
 Documentation/git.txt           |  7 ++++
 Documentation/gitattributes.txt |  5 +++
 diff.c                          | 62 ++++++++++++++++++++++++---------
 t/t4020-diff-external.sh        | 44 +++++++++++++++++++++++
 userdiff.c                      |  8 +++--
 userdiff.h                      |  7 +++-
 7 files changed, 128 insertions(+), 19 deletions(-)

-- 2.45.2