(comint-previous-input): Don't clobber input line when moving off either

(comint-previous-input): Don't clobber input line when moving off either
end of the input history ring.
(comint-delete-input): New function, used by `comint-previous-input' and
others.
(comint-previous-matching-input): Use `coming-delete-input'. Save the
partial input if leaving the edit line.  Goto point-max before deleting
input to avoid partial input fragments hanging around.
(comint-restore-input): New function, used by `comint-previous-input',
and bound to "C-c C-j".