branch: rework comments for future developers

branch: rework comments for future developers

For two cases in which we do not explicitly pass --track=<choice>
option down to the submodule--helper subprocess, we have comments
that say "we do not have to pass --track", but in fact we not just
do not have to, but it would be incorrect to pass any --track option
to the subprocess (instead, the correct behaviour is to let the
subprocess figure out what is the appropriate tracking mode to use).

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>