Deprecating git diff ..; dealing with other ranges

Hello all,

I was in the process of deprecating `git diff <commit>..<commit>` as discussed here[1]. However, I ran into a weird case that I'm not sure how to deal with.

In t3430-rebase-merges.sh:382, we have the following test case which invokes git diff:

	test_expect_success 'with --autosquash and --exec' '
		git checkout -b with-exec H &&
		echo Booh >B.t &&
		test_tick &&
		git commit --fixup B B.t &&
		write_script show.sh <<-\EOF &&
		subject="$(git show -s --format=%s HEAD)"
=>		content="$(git diff HEAD^! | tail -n 1)"
		echo "$subject: $content"
		EOF
		test_tick &&
		git rebase -ir --autosquash --exec ./show.sh A >actual &&
		grep "B: +Booh" actual &&
		grep "E: +Booh" actual &&
		grep "G: +G" actual
	'

It gets caught in my attempt to only deprecate ..'s. Technically, it's undocumented behaviour and it only happens to work because git-diff accept ranges but it doesn't operate in an intuitive way.

I was just wondering what we should do about this case? Should we deprecate all invocations of `git diff <range>` except for the special case of `git diff <commit>...<commit>`, or should we _only_ deprecate `git diff <commit>..<commit>` and allow all other forms of ranges, even though it was undocumented behaviour?

Thanks,

Denton

[1]: https://public-inbox.org/git/xmqqmumy6mxe.fsf@gitster-ct.c.googlers.com/