`*` incorrectly restores view due to wrapped lines

### Problem

If e.g. 1) the first line in the window is wrapped, 2) the cursor is on any line below that, and 3) `*` would cause the view to move down (i.e. the next match is not yet visible), then [`winline() - s:winline`](https://github.com/junegunn/vim-slash/blob/31aee09b7ea8893a18fa34f65e63e364fc998444/plugin/slash.vim#L46) will usually compute an incorrect `sdiff`, and we will `` more times than necessary, shifting the view up by at least 1 line (unless we were already at the beginning of the buffer, or there are some other edge cases).

### Possible solution

I believe using `winsaveview()['topline']` instead of `winline()` avoids this discrepancy between buffer lines and screen lines.

I'll put up a PR with that approach, but 1) I'm not sure if there's a simpler way to get `topline` (maybe some function I overlooked?), and 2) although I've tested this with all edge cases that I'm aware of, I can't be sure that this entirely fixes this issue, or that it doesn't break anything else.