cocci: avoid "should ... be a metavariable" warnings

Since [1] running "make coccicheck" has resulted in [2] being emitted
to the *.log files for the "spatch" run, and in the case of "make
coccicheck-test" we'd emit these to the user's terminal.

Nothing was broken as a result, but let's refactor the relevant rules
to eliminate the ambiguity between a possible variable and an
identifier.

1. 0e6550a2c6 (cocci: add a index-compatibility.pending.cocci,
   2022-11-19)
2. warning: line 257: should active_cache be a metavariable?
   warning: line 260: should active_cache_changed be a metavariable?
   warning: line 263: should active_cache_tree be a metavariable?
   warning: line 271: should active_nr be a metavariable?

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

contrib/coccinelle/index-compatibility.cocci

@@ -1,22 +1,26 @@
 // the_index.* variables
 @@
+identifier AC = active_cache;
+identifier ACC = active_cache_changed;
+identifier ACT = active_cache_tree;
 @@
 (
-- active_cache
+- AC
 + the_index.cache
 |
-- active_cache_changed
+- ACC
 + the_index.cache_changed
 |
-- active_cache_tree
+- ACT
 + the_index.cache_tree
 )
 
 @@
+identifier AN = active_nr;
 identifier f != prepare_to_commit;
 @@
   f(...) {<...
-- active_nr
+- AN
 + the_index.cache_nr
   ...>}