lib-submodule-update: prepend "git" to $command

lib-submodule-update: prepend "git" to $command

Since all invocations of test_submodule_forced_switch() are git
commands, automatically prepend "git" before invoking
test_submodule_switch_common().

Similarly, many invocations of test_submodule_switch() are also git
commands so automatically prepend "git" before invoking
test_submodule_switch_common() as well.

Finally, for invocations of test_submodule_switch() that invoke a custom
function, rename the old function to test_submodule_switch_func().

This is necessary because in a future commit, we will be adding some
logic that needs to distinguish between an invocation of a plain git
comamnd and an invocation of a test helper function.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>