advice: add --no-advice global option

advice: add --no-advice global option

Advice hints must be disabled individually by setting the relevant
advice.* variables to false in the Git configuration. For server-side
and scripted usages of Git where hints can be a hindrance, it can be
cumbersome to maintain configuration to ensure all advice hints are
disabled in perpetuity. This is a particular concern in tests, where
new or changed hints can result in failed assertions.

Add a --no-advice global option to disable all advice hints from being
displayed. This is independent of the toggles for individual advice
hints. Use an internal environment variable (GIT_ADVICE) to ensure this
configuration is propagated to the usage site, even if it executes in a
subprocess.

Signed-off-by: James Liu <james@jamesliu.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>