if someone logs a line longer than 65*1024 bytes before it contains a newline, the default ScanLines func in bufio.Scanner can't handle it. Copy that func and add a conditional to clear buffer and continue scanning in this situation.

Uses `fmt.Sprint()` instead of `fmt.Sprintf()` for log deduplication.

Fixes: #6796

The change done in #3964 causes the if statement to check if debug logging should be enabled to always pass.