[RFC PATCH 0/3 UPDATED] diff --check enhancements

[1/3] Use "whitespace" consistently

Same.

[2/3] "diff --check" should affect exit status

Additional tests added, and one correction (git diff-tree was only
setting the exit status if the --stdin option was passed).

[3/3] New version of pre-commit hook

A demonstration of the benefit of [2/3]; the pre-commit hook
effectively becomes a one-liner.