* comint.el (comint-previous-input): Don't clobber input line

* comint.el (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-g].