reset: replace '--quiet' with '--no-refresh' in performance advice

reset: replace '--quiet' with '--no-refresh' in performance advice

Replace references to '--quiet' with '--no-refresh' in the advice on how to
skip refreshing the index. When the advice was introduced, '--quiet' was the
only way to avoid the expensive 'refresh_index(...)' at the end of a mixed
reset. After introducing '--no-refresh', however, '--quiet' became only a
fallback option for determining refresh behavior, overridden by
'--[no-]refresh' or 'reset.refresh' if either is set. To ensure users are
advised to use the most reliable option for avoiding 'refresh_index(...)',
replace recommendation of '--quiet' with '--[no-]refresh'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>