submodule: refrain from filtering GIT_CONFIG_COUNT

submodule: refrain from filtering GIT_CONFIG_COUNT

14111fc492 ("git: submodule honor -c credential.* from command line",
2016-03-01) taught Git to pass through the GIT_CONFIG_PARAMETERS
environment variable when invoking a subprocess on behalf of a
submodule. But when d8d77153ea ("config: allow specifying config entries
via envvar pairs", 2021-01-15) introduced support for GIT_CONFIG_COUNT
(and its associated GIT_CONFIG_KEY_? and GIT_CONFIG_VALUE_?), the
subprocess mechanism wasn't updated to also pass through these
variables.

Since they are conceptually the same (d8d77153ea was written to address
a shortcoming of GIT_CONFIG_PARAMETERS), update the submodule subprocess
mechanism to also pass through GIT_CONFIG_COUNT.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>