advice: allow disabling default branch name advice

advice: allow disabling default branch name advice

The default branch name advice message is displayed when
`repo_default_branch_name()` is invoked and the `init.defaultBranch`
config is not set. In this scenario, the advice message is always shown
even if the `--no-advice` option is used.

Adapt `repo_default_branch_name()` to allow the default branch name
advice message to be disabled with the `--no-advice` option and
corresponding configuration.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>