[PATCH 0/4] diff dry-run cleanups

Since we were talking about this topic so much this week, I wanted to collect these patches before they fell out of my head. They would go on top of ly/diff-name-only-with-diff-from-content.

  [1/4]: diff: send external diff output to diff_options.file

    This one is actually a bug-fix that we found along the way, and
    could be applied completely independently. AFAICT the bug has been
    there since 2008, though, so probably not super urgent. ;)

  [2/4]: diff: drop save/restore of color_moved in dry-run mode

    This one is hopefully a no-brainer simplification.

  [3/4]: diff: replace diff_options.dry_run flag with NULL file
  [4/4]: diff: drop dry-run redirection to /dev/null

    And then these ones clean up our extra layer of fixes, converting
    any missed dry_run checks into segfaults. But we hope there aren't
    any after Lidong's fixes. :) So possibly controversial, but I think
    the end result is nicer to read.

 diff.c                   | 56 +++++++++++-----------------------------
 diff.h                   |  2 --
 t/t4020-diff-external.sh | 10 +++++++
 3 files changed, 25 insertions(+), 43 deletions(-)

-Peff