Lots of internal functions in submodule-confic.c have a first parameter
`struct submodule_cache *cache`, which currently always refers to the
global variable `cache` in the file. To avoid confusion rename the
global `cache` variable.
Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>