advice: Add "all" option to disable all hints

Advice hints must be disabled individually by setting the relevant advice.* variables to false in the Git configuration. For server-side usages of Git where hints aren't necessary, it can be cumbersome to maintain configuration to disable all advice hints. This is especially the case if/when new advice hints are added.

Add a new "all" advice variable which acts as a toggle for all advice types. This takes precedence over any other advice type.