bug#7567: Please add a history variable to read-regexp

[Juri Linkov]

> Please take a look in the current code in query-replace-read-from.
> This currently does not use read-regexp (which it should) because it
> is easier to switch history variable if you don't.

`query-replace' should share its history with `from' and `to' parts by default
as it does now.  But I agree that it could provide the same defaults
as `read-regexp'.