magit.el: fix keybinding for toggle-gpgsign

6fad2b11 (introduce commit filed for GPG signature, 2013-03-17)
introduced magit-log-edit-toggle-gpgsign and bound it to "C-c C-S",
without realizing that it's equivalent to "C-c C-s", the keybinding
for magit-log-edit-toggle-signoff.  As a result, it's impossible to
toggle signoff with a keybinding now.  Fix this by changing the
keybinding for toggling gpgsign to "C-c C-g".

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>