Fix scrolling when header is present

[nicula-stripe]

Reproducing the bug

• Apply the following patch on a clean master:

diff --git a/example/cli/main.go b/example/cli/main.go
index 7b9b8d2..3baf694 100644
--- a/example/cli/main.go
+++ b/example/cli/main.go
@@ -29,7 +29,8 @@ func main() {
                slice,
                func(i int) string {
                        return slice[i]
-               })
+               },
+               fuzzyfinder.WithHeader("header"))
        if err != nil {
                log.Fatal(err)
        }

• Run printf 'a\nb\nc\nd\ne\nf' | go run example/cli/main.go.
• Reduce the height of the terminal such that not all items fit on the screen.
• Scroll all the way up in the list and notice how the cursor is one position above what you can actually see. Demo:

bug.mov

Fixing the bug

If the prompt has a header, increment the number of already-occupied lines (the other ones being the line with the search input, and the line with the result count).