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 define
"USE_THE_INDEX_VARIABLE" in the individual test helpers that need
it. This mirrors how we do the same thing in the "builtin/" directory.

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

t/helper/test-cache-tree.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "tree.h"

t/helper/test-dump-cache-tree.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "tree.h"

t/helper/test-dump-split-index.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "split-index.h"

t/helper/test-dump-untracked-cache.c

@@ -1,4 +1,4 @@
-#define USE_THE_INDEX_COMPATIBILITY_MACROS
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "dir.h"

t/helper/test-fast-rebase.c

@@ -10,7 +10,7 @@
  * refactoring is the better route).
  */
 
-#define USE_THE_INDEX_COMPATIBILITY_MACROS
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 
 #include "cache-tree.h"

t/helper/test-lazy-init-name-hash.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "parse-options.h"

t/helper/test-read-cache.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "config.h"

t/helper/test-scrap-cache-tree.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "lockfile.h"

t/helper/test-tool.h

@@ -1,7 +1,6 @@
 #ifndef TEST_TOOL_H
 #define TEST_TOOL_H
 
-#define USE_THE_INDEX_COMPATIBILITY_MACROS
 #include "git-compat-util.h"
 
 int cmd__advise_if_enabled(int argc, const char **argv);

t/helper/test-write-cache.c

@@ -1,3 +1,4 @@
+#define USE_THE_INDEX_VARIABLE
 #include "test-tool.h"
 #include "cache.h"
 #include "lockfile.h"