CodingGuidelines: allow declaring variables in for loops

CodingGuidelines: allow declaring variables in for loops

Since 44ba10d6712 (revision: use C99 declaration of variable in for()
loop, 2021-11-14) released with v2.35.0 we've had a variable declared
with in a for loop.

Since then we've had inadvertent follow-ups to that with at least
cb2607759e2 (merge-ort: store more specific conflict information,
2022-06-18) released with v2.38.0.

As November 2022 is within the window of this upcoming release,
let's update the guideline to allow this.  We can have the promised
"revisit" discussion while this patch cooks, and drop it if it turns
out that it is still premature, which is not expected to happen at
this moment.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>