bug#52751: 27.1; Go back to editing the from part in query replace

[Juri Linkov]

> If you have two input fields which you can edit before submitting then the 
> whole arrow
> thing is obsolete, because you don't have a single field where you have to 
> insert
> a marker to separate the from/to inputs.

Unfortunately, there are problems with two fields.

First, each of these fields should act as the minibuffer
where M-p and M-n navigates the history of previous FROM->TO pairs.

Second, typing RET in the first field is ambiguous: it can mean either
that the user want to move to the second field, or that the user want
to replace all matches with an empty string by leaving the second field
empty.  This requires special-casing: RET in the first field to move to
the second field, and RET in the second field to accept both fields.