cocci: add a index-compatibility.pending.cocci

cocci: add a index-compatibility.pending.cocci

Add a coccinelle rule which covers the rest of the macros guarded by
"USE_THE_INDEX_COMPATIBILITY_MACROS" cache.h. If the result of this
were applied it can be reduced down to just:

	#ifdef USE_THE_INDEX_COMPATIBILITY_MACROS
	extern struct index_state the_index;
	#endif

But that patch is just under 2000 lines, so let's first add this as a
"pending", and then incrementally pick changes from it in subsequent
commits. In doing that we'll migrate rules from this
"index-compatibility.pending.cocci" to the "index-compatibility.cocci"
created in a preceding commit.

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