fix new reverse search behavior when existing buffer is non-empty

[KristofferC]

Before this would open the reverse search with an empty buffer and append the selected result to the pre-existing buffer.

Now, it uses the input buffer as an input to the reverse search instead and it clears the original buffer when inserting the result.

[tecosaur]

Reading through the changes, nothing jumps out at me as looking fishy 👍

I'll give it another look/test tomorrow (or feel free to merge it if you're confident it's fine).

[rfourquet]

I'm not sure I prefer this fix. I can imagine starting to type some input, and realize I want to also insert some history item, in which case the current behavior is perfect.
On using the current input buffer as search input: this was not the behavior of 1.12, and this could easily achieved by killing the line (C-u) and pasting it into the search input (C-y, but it's not implemented apparently).

[KristofferC]

On using the current input buffer as search input: this was not the behavior of 1.12,

Huh? Okay, it is quite confusing because it (the 1.12 search) shows the existing input in the search window after pressing Ctrl-R but it doesn't search for it. And then when you select something, it does not append to it.

However, other reverse searches I've used (like fzf) have this behavior (the one in this PR).

[tecosaur]

I often invoke C-r before typing anything out, so I don't have a clear sense of what the "right" behavior here is.

I will say that C-u at least makes it easy to clear the input, if it's prefilled. That said, it's not a keybinding I think many people are familiar with?