cocci & cache.h: remove rarely used "the_index" compat macros

Since 4aab5b46f4 (Make read-cache.c "the_index" free., 2007-04-01)
we've been undergoing a slow migration away from these macros, but
haven't made much progress since f8adbec9fe (cache.h: flip
NO_THE_INDEX_COMPATIBILITY_MACROS switch, 2019-01-24).

Let's move forward a bit by changing the users of those macros that
are rare enough that we can convert them in one go, and then remove
the compatibility shim.

The only manual change to the C code here is to "cache.h", the rest is
all the result of applying the new "index-compatibility.cocci".

Even though it's a one-off, let's keep the coccinelle rules for
now. We'll extend them in subsequent commits, and this will help
anything that's in-flight or out-of-tree to migrate.

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

builtin/am.c

@@ -1930,7 +1930,7 @@ static void am_resolve(struct am_state *state, int allow_empty)
 		}
 	}
 
-	if (unmerged_cache()) {
+	if (unmerged_index(&the_index)) {
 		printf_ln(_("You still have unmerged paths in your index.\n"
 			"You should 'git add' each file with resolved conflicts to mark them as such.\n"
 			"You might run `git rm` on a file to accept \"deleted by them\" for it."));
@@ -2045,7 +2045,7 @@ static int clean_index(const struct object_id *head, const struct object_id *rem
 	if (!remote_tree)
 		return error(_("Could not parse object '%s'."), oid_to_hex(remote));
 
-	read_cache_unmerged();
+	repo_read_index_unmerged(the_repository);
 
 	if (fast_forward_to(head_tree, head_tree, 1))
 		return -1;