Function: comint-get-old-input-default

Default for `comint-get-old-input'.
If `comint-use-prompt-regexp' is nil, then either
return the current input field, if point is on an input field, or the
current line, if point is on an output field.
If `comint-use-prompt-regexp' is non-nil, then return
the current line with any initial string matching the regexp
`comint-prompt-regexp' removed. (fn)