CodingGuidelines: give deadline for "for (int i = 0; ..."

We raised the weather balloon to see if we can allow the construct
in 44ba10d6 (revision: use C99 declaration of variable in for()
loop, 2021-11-14), which was shipped as a part of Git v2.35.
Document that fact in the coding guidelines, and more importantly,
give ourselves a deadline to revisit and update.

Let's declare that we will officially adopt the variable declaration
in the initializaiton part of "for ()" statement this winter, unless
we find that a platform we care about does not grok it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -217,7 +217,10 @@ For C programs:
    the first statement (i.e. -Wdeclaration-after-statement).
 
  - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
-   is still not allowed in this codebase.
+   is still not allowed in this codebase.  We are in the process of
+   allowing it by waiting to see that 44ba10d6 (revision: use C99
+   declaration of variable in for() loop, 2021-11-14) does not get
+   complaints.  Let's revisit this around November 2022.
 
  - NULL pointers shall be written as NULL, not as 0.