emacs: magit: advise magit-commit to use the --gpg-sign switch by default
emacs: magit: advise magit-commit to use the --gpg-sign switch by default

Also advise related functions magit-commit-amend, magit-commit-extend,
and magit-commit-reword.

Moreover, offer the --no-gpg-sign switch as a fallback solution.

Hopefully this completely solves GPG signing in Magit.

zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles