Since the documentation asserts that the emitted key/value lines will
be sorted lexicographically, can we also have a test that verifies
that behavior?

Since we agreed to return the values in the order they were requested,
I'll test that order instead:

test_expect_success 'the values are returned in the same order they were requested' '
	printf "references.format=files\nlayout.bare=false\n" >expected1 &&
	printf "layout.bare=false\nreferences.format=files\n" >expected2 &&
	git init --ref-format=files same-order &&
	git -C same-order repo info references.format layout.bare >actual1 &&
	git -C same-order repo info layout.bare references.format >actual2 &&
	test_cmp expected1 actual1 &&
	test_cmp expected2 actual2
'