cocci & cache.h: apply variable section of "pending" index-compatibility

cocci & cache.h: apply variable section of "pending" index-compatibility

Mostly apply the part of "index-compatibility.pending.cocci" that
renames the global variables like "active_nr", which are a shorthand
to referencing (in that case) a struct member as "the_index.cache_nr".

In doing so move more of "index-compatibility.pending.cocci" to
"index-compatibility.cocci".

In the case of "active_nr" we'd have a textual conflict with
"ab/various-leak-fixes" in "next"[1]. Let's exclude that specific case
while moving the rule over from "pending".

1. 407b94280f8 (commit: discard partial cache before (re-)reading it,
   2022-11-08)

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>