Re: [PATCH 10/12] cache.h & test-tool.h: add & use "USE_THE_INDEX_VARIABLE"

> In a preceding commit we fully applied the
> "index-compatibility.pending.cocci" rule to "t/helper/*". Let's now
> stop defining "USE_THE_INDEX_COMPATIBILITY_MACROS" in test-tool.h
> itself, and instead instead move the define to the individual test
> helpers that need it. This mirrors how we do the same thing in the
> "builtin/" directory.
>
> We could start using "USE_THE_INDEX_COMPATIBILITY_MACROS", but let's
> instead introduce a narrow version of it named
> "USE_THE_INDEX_VARIABLE". As the name suggests this doesn't give us
> any of the compatibility macros, but only "the_index" variable.
>
> Eventually we'll move other outstanding users of
> "USE_THE_INDEX_COMPATIBILITY_MACROS" over to using this more narrow
> and descriptive define.