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

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. 0e6550a2c63 (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>