bug#3746: M-r in comint mode should use isearch

[Stefan Monnier]

> Please see the attached code that implements isearch in comint mode.

What would be the "UI" (e.g. key-binding) for it?
Only the comint-history-isearch defcustom?

> Maybe we should change `comint-previous-input' and `comint-next-input'
> to signal an error at the beginning/end of the history ring?

That sounds fine to me, but pleast double check all uses of those
functions to make sure it won't introduce other problems.


        Stefan