Restore Ctrl-Backspace in comment editor

Some users expect Ctrl-Backspace to delete the last word.
By overriding it as an alias for Ctrl-d (discard comment) we not
only took away a useful feature from a user, but we also made it
possible to lose unsaved comments by an accidental keystroke.

Instead, only support Esc (to discard a new comment) or Ctrl-d
(to discard a new or a saved comment).

Signed-off-by: Shawn O. Pearce <sop@google.com>