emacs: better fix for god-mode undo in c-mode

undo didn't work in god-mode because in c-mode "/" is bound to
c-electric-slash, not self-insert-command. i had fixed this by mapping
"/" to undo in god-local-mode-map, overriding the major mode binding.

a better fix is to remap c-electric-slash to god-mode-self-insert. this
is what god mode does with self-insert-command.

also remap sh-assignment to god-mode-self-insert, and add a comment
explaining it. I'm sure more remaps will be added here in the future.